Instead of the predictable "here!" or, "present!", or if you're asked to give a fun fact about yourself, just stand up, casually pull the ice bucket out of your school bag, and announce: "Hi my name is (insert name) and this is my ALS ice bucket challenge." Be sure to watch this video to make sure you get it right!
This is definitely memorable, but make sure to bring extra clothes to change in, as your teacher will probably dismiss you to change, if not send you home right then and there. Your classmates will love that you're so thoughtful towards charity, that you'd do anything to avoid donating.