Call eight male and eight female basketball players to the floor. Have the boys sit in chairs facing a girl. Blindfold the girls and have them put makeup on the males for hilarious results.
Cover the floor in white roll paper. Tape it securely so that no floor is showing. Assign each class a color and supply paint. Tell them you're going to blow a whistle; when they hear it, they have one minute to cover as much of the floor as possible in their class's color.
Hold your pep rally outside on concrete. Supply sidewalk chalk and split students into groups of 10 to create a colorful tribute to the school.
Split students into teams of five -- or ask for volunteers -- and have an improv day. Or, write down the names of a few dozen popular movies on strips of paper. Ask for five volunteers to come to the floor and draw a strip. Tell them they have one minute to act out a scene from the movie so that the audience can guess what it is. You could do the same thing but with teachers' names. Have each student pick a name and do her best impersonation of the teacher. Whoever is first to correctly guess who the student is impersonating gets a small prize.
Take your pep rally to the football field and hold an Easter egg hunt -- even if it's not April. Hide eggs all over the field, bleachers and grounds. The class to collect the most eggs wins a prize, such as a pizza party.
Have students sign up in teams and have a tug 'o war tournament.
Hold a teacher dance-off. Bring two teachers out and have them dance to silly music, then let the crowd vote on the best by round of applause. The loser goes back to his seat and another teacher comes to compete against the remaining dancer.
Supply toilet paper and ask for teacher volunteers. Call two students and have them race to see who can wrap toilet paper around a teacher the fastest.
Or, have all students take off their shoes and throw them into a giant pile on the floor. Have teachers mix them up. Blow a whistle and let students converge on the pile and try to find their shoes. Play music during the scramble.
Hold a male beauty pageant. Peruse a thrift store for dated prom dresses and have each class vote for their pageant contestant. At the pep rally, have the guys arrive in the gowns and fully made-up. Let them choose a silly talent to show off. Choose three teachers to serve as judges and award a silly prize, like a can of hairspray, to the winner.