There are many occasions that require banquets in a high school setting. Any or all of these may come under your job description. The most important thing to find out initially, is how many people will be attending. This will be a large factor in determining where you will hold the dinner. The location you choose will largely determine the atmosphere of the event.
Most schools try to keep costs down, and often have pot-luck dinners for sports teams.
Whatever the group wants to spend will also determine how fancy your dinner will be. Get a feel for the group's ideas about their banquet by asking them lots of questions. The more you know about their ideas, the easier it will be to plan an event that will be successful. As a result, they may be a difficult audience to satisfy. Part of your job will be to educate them as to what to expect.