They need to be in at least two days before the absence. For unforseen absences, you get two days for every day you're absent up to a total of six days to make up the work. Parties are held after almost every football games and some basketball games. Other parties are held about once a month. Dress for school parties and dances is casual except for homecoming, winter formal, and prom.
Homecoming is a little dressier than church.
For Winter Formal, most guys get tuxes or at least wear suits, and the girls wear formal dresses, but usually not gowns. Guys wear tuxes to prom, and girls often wear gowns or really really fancy dresses.