I haven't been in school in nearly a decade but I was performing professionally when I was so here's my tips.
1) approach and perform for anyone that will stand still long enough to see you.
Reason: You never know who is watching you and what connections they have. Your 5 minute little cards across rutine could land you a $500 dollar christmas party gig because Britany's daddy is a CEO executive.
2) Do effects that require little to no setup, or tear down with as little time as possible between resets.
3) Don't worry too much about hecklers. If you learn to control your audience you'll learn to control the hecklers.
4) Cards, coins, mentalism is the best things to do. They pack small and play big. Learn to do the following effects and ALWAYS keep them with you.
1_ Cards Across
2_ Triumph
3_ Ambitious
4_ Copper, Silver, Brass, Freaky, or Hopping Halvs
5_ Hundy 500
6_ Loops
7_ Hot Rods
5) Someone else mentioned using props that can be examined by your spectators and for a high school I would say this is important.
Hope this helps