Legally speaking, you can get done over for using the school colours and logo as it's clearly deceptive practice, unless you get the school's permission.l
Putting posters up in a way that they can't be removed (i.e. cornflour slime paste) will get you done for vandalism, and potentially damage-to-property, as well as trespass if you didn't have permission.
With your own school you could probably quite easily get permission if you had a business studies type class, and convinced the teacher that you were trying to do this as a case study / extra credit assignment / general entrepreneurial thingy that school pride has instilled in you. Teachers love the "I have school pride" shit. Then, when they ok it, you just have to go to the principal / headmaster and make sure they're cool with it too.
Then at least you have one school that's ok with you doing it.
Have you considered doing a short MMS/SMS campaign? They go viral pretty quickly if you send it out to 10 people that you KNOW love to forward shit on.
Also, do you know if your friends phones have the ability to use QR codes?
If so, you may want to put the QR code on the poster / banner that you put up, so they can just go straight to the site from their phones.
Because I'm a nice guy, I've attached the QR Code for your site (works on my phone at least, anyone else with QR enabled want to test it?).
Best thing about QR codes is that you can just paste them up anywhere, and people that understand what they're for will generally load them up just to see what it is.