I just had a look on the EV Website and they don't have any information for Victorian Interschools 2011 on at the moment. If it's anything like IEQ (Interschool Equestrian Queensland), then you'll need to become a member of EA (Junior Fees for Vic are $155), and get your school to pay a fee of around $50 and send some forms away. You'll then need to register both you and your horse for Interschool Vic and decide on a school uniform, which must be registered. You should get either a bridle number or a saddle cloth number for you and your horse. You can generally have more than one horse registered and you will have a different number for both. Your horse does not have to be registered with EV, as long as it's registered with interschools. The rules are pretty much the same as general EV competitions except, you wear your school's riding uniform instead of what you'd usually wear. Most normal EV events will either have classes set aside for Interschool riders or you'll be able to enter normal EV classes but with your interschool number on. As for straight interschool competitions, I'm not sure =/ Sorry.
With the lack of information online, I recommend ringing up EV or finding out who the coordinator for Vic Interschools is and ringing them. Disciplines
^ That's the website for it. I looked in the forms section, but couldn't find any. EA Interschools
^ That's the website for EA Interschools. It doesn't have much information on it but should give you a background. Interschool
^ That's the website for Qld Interschools. I know it's not the right state but it should give you some idea of what should happen.
I hope I've helped a bit, and good luck with getting registered and all that! Interschool comps are generally a lot of fun and you make quite a few friends. =]
Oh, and I forgot to add that you can find all of the EV rules on the EV website that I gave you (with the interschool stuff on it). Just follow the links.