Although everyone is telling you so far to get out and do what you should to get a voice across, you have to understand that plastering up flyers and posters and paraphernalia does get to some people. I know all to well about getting sent up to the principals office day after day due to my 'activism'.
If I were you and this is coming from exprience, (i promise) start by talking to people you know that can make a difference. If you have a teacher you know of that supports being cruelty free and veg chat with them; they have much more of an influence on the school board and staff body then you will have (another thing learnt via experience). At school there's a ranking system, principal/staff/students. If you can get things across via staff; students and higher powers will eventually see them and have to act upon them.
Your cafeteria is a good place to start. If you're having a fete or a carnival offer to do the BBQ and cook only soy/vegan products and don't tell anyone the difference. The look on their faces and their reactions will be priceless once and when they find out.
To end this though, please note that if you do decided to not follow anything I've said or take it into account that although people may or may not react to pictures you plaster up, which can be as grotesque or placid as you please; some people are taught and brought up the way they are due to cultural or religious reasons. You have no right to force your beliefs onto them; just the same way they have no right to do the same to you. Don't be forceful or manipulative; simply make people aware; they'll be curious for sure, and once they are - they'll come to you for more understandings.
As someone's already said before; the best place to start is via reports, stand up aural presentations, media files, try bulk emailing people; or be sneaky and change all the computer backgrounds in the library... i meann uhh.....