Question: How long is each half of a school field hockey game?
About the article "Field hockey", section "Local rules".

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Other games derived from hockey or its predecessors include the following:  Air hockey is played indoors with a puck on an air-cushion table.
Beach hockey, a variation of street hockey, is a common sight on Southern California beaches.
Ball hockey is played in a gym using sticks and a ball, often a tennis ball with the fuzz removed.
Bandy is played with a ball on a football field-sized ice arena, typically outdoors, and with many rules similar to association football  Box hockey is a school yard game played by two people.
The object of the game is to move a hockey puck from the center of the box out through a hole placed at the end of the box (known as the goal).
Each player kneels and faces one another on either side of the box, and each attempts to move the puck to the hole on their left.
Broomball is played on an ice hockey rink, but with a ball instead of a puck and a "broom" (actually a stick with a small plastic implement on the end) in place of the ice hockey stick.
Instead of using skates, special shoes are used that have very soft rubbery soles to maximize grip while running around.
Deck hockey is traditionally played by the Royal Navy on the ships' decks, using short wooden 'L' shaped sticks.
Floor hockey is a form of hockey played on foot,on flat, smooth floor surface.
It is usually played inside in gymnasiums and such.
Floorball, is a form of hockey played in a gymnasium or in sport halls.
A whiffle ball is used instead of a plastic ball, and the sticks are made from composite materials.
The sticks are only one meter long.
Foot hockey or sock hockey is played using a bald tennis ball or rolled up pair of socks and using only the feet.
It is popular at elementary schools in the winter.
Gym hockey is a form of ice hockey played in a gymnasium.
It uses sticks with foam ends and a foam ball or a plastic puck.
Hurling and Camogie are Irish games bearing some resemblance to – and notable differences from – hockey.
Indoor field hockey is an indoor variation of field hockey.
Mini hockey In the United States is a form of hockey (also known as "mini-sticks") which is played in basements of houses.
Players get down on their knees, using a miniature plastic stick, usually about 15 inches (38 cm) long to maneuver a small ball or a soft, fabric covered mini puck into a miniature goals.
In England 'mini hockey' refers to a seven-a-side version of field hockey, played on an area equivalent to half a normal pitch for younger players  Nok Hockey is a table-top version of hockey played with no defense and a small block in front of the goal.
Power hockey is a form of hockey for persons requiring the use of an electric (power) wheelchair in daily life.
PowerHockey is a competitive sports opportunity for the physically disabled.
Ringette is an ice hockey variant that was designed for female players; it uses a straight stick and a rubber ring in place of a puck.
"Note: Ringette distances itself from hockey as it has its own set of rules and is closely related to a mix of lacrosse and basketball."
Rinkball is a Scandinavian team sport, played in an ice hockey rink with a ball.
Rossall hockey is a variation played at Rossall School on the sea shore in the winter months.
Its rules are a mix of field hockey, rugby and the Eton wall game.
Shinny is an informal version of ice hockey.
Shinty is a Scottish game now played primarily in the Highlands  Skater hockey is a variant of inline hockey, played with a ball.
Spongee is a cross between ice hockey and broomball and is most popular in Manitoba, Canada.
A stick and puck are used as in hockey (the puck is a softer version called a "sponge puck"), and the same soft-soled shoes used in broomball are worn.
The rules are basically the same as ice hockey, but one variation has an extra player on the ice called a "rover".
Table hockey is played indoors with a table-top game.
Underwater hockey is played on the bottom of a swimming pool.
Unicycle hockey is played on a hard surface using unicycles as the method of player movement.
There is generally no dedicated goalkeeper.  
An umpire in field hockey is a person with the authority to make decisions on a hockey field in accordance with the laws of the game.
Each match is controlled by two such umpires, where it is typical for umpires to aid one another and correct each other when necessary . 
Webster Groves High School sponsors a number of different sports, including football (men's), field hockey, soccer (men's and women's), basketball (men's and women's), baseball (men's), softball (women's), golf (men's and women's), track and field (men's and women's), swimming (men's and women's), and lacrosse (women's).
Ice hockey and men's lacrosse are club sports at the high school.
The "Turkey Day football game" is an annual game held on Thanksgiving Day between Webster Groves High School and its rival, Kirkwood High School.
The rivalry between the two schools is the oldest current Thanksgiving Day rivalry west of the Mississippi River.
The location of the game alternates each year between Webster’s Moss Field and Kirkwood’s Lyon’s Memorial field.
A number of festivities surround the game, including a shared dance and a separate bonfire and pep rally at each school.
2007 was the 100th year of this storied series between the two schools, and the game had attendance exceeding 12,000 fans. 
Indoor hockey developed in Germany during the 1950s, quickly spreading to other European nations.
Belgium was one of the countries to adopt the field hockey variant, and in 1966 René Frank, a native of Belgium, who was later to become President of the FIH, persuaded the German Hockey Associations to give responsibility over the rules of Indoor Hockey to the FIH.
This led to the FIH recognising indoor hockey in its constitution in 1968.
Whereas in many countries field hockey is played all year long, in Germany and Austria the hockey season is divided evenly into a field hockey half in summer and an indoor hockey season in Winter.
There has been criticism that this impairs these countries' chances in international field hockey competition, but on the other hand the north European climate favours indoor hockey in Winter, as outdoor pitches may be unplayable due to snow and ice.
In Germany's case one could argue that it complements and enhances skills of their players with the German Women winning gold in Athens Olympics and German Men winning the 2006 Field Hockey World Cup, the 2007 indoor hockey world cup and gold at the 2008 Beijing Olympics.
In Germany indoor hockey is quite popular with many players, as due to the smaller fields and the use of side-boards the game-play is much faster.
It is also both technically and physically very demanding.
The first FIH sanctioned tournament matches of Indoor Hockey were played in 1972. 
Indoor field hockey, or indoor hockey, is an indoor variant of "traditional" outdoor field hockey.
It is not to be confused with other indoor hockey variants such as rink hockey or floorball.
It is traditionally and mainly played as a pastime by outdoor field hockey players during the off-season, when the outdoor pitches are frozen, or alternatively conditions are too hot for outdoor play.
Indoor field hockey is played in regular national and international championships.
The first Indoor Field Hockey World Cup was organized in 2003.
It included countries which do not compete at the highest level of the outdoor game.
Indoor field hockey differs from its outdoor parent in several ways:  The playing field is smaller than the outdoor pitch.
An indoor pitch is 18m to 22m wide by 36m to 44m long, divided by a center line.
The shooting circle is a semicircle measured out 9m from each goal post.
Side-boards mark the sidelines, helping to keep the ball in the field and allowing players to play against it.
Despite who hit it, when the ball goes over the endline the defensive team will receive a hit coming out.
The pitch is made of wood or synthetic material.
The board surface facing inwards to playing surface is angled slightly to encourage ball to bounce down not up.
The goals are smaller than a field hockey: 2m high by 3m wide and a minimum of 1m deep.
(This is the same size as Team Handball goals as early indoor hockey used existing handball courts)  A team consists of 5 players on the pitch, 4 field players and 1 goalkeeper, with a maximum 12 players on a team.
Internationally the game is divided into 2 periods of 20 minutes.
(In the German Indoor league, they play 30 minute halves with the ability to call time outs as in basketball.)
Halftime break is 15 minutes.
In case of a regulation tie teams play up to two 5-minute golden-goal overtimes; otherwise, the game will be decided by penalty strokes (5 rounds for each team followed by sudden-death rounds in case of ties).
The players may not hit the ball, but only push it or deflect it, and may not raise the ball except in the shooting circle, with the purpose of scoring a goal.
The balls and the sticks are similar, but players prefer lighter sticks than for the outdoor game.
The small field and sideboards make indoor field hockey a quick, technical and physical game.
Some of the original rules eventually influenced outdoor hockey, such as unrestricted substitution.
It is often an ideal game for field hockey players to develop vision on and off the ball, developing a better understanding of tactics and set plays.