One of the most important things you can do is to get your cabling right now. Put in more than you need currently. Keep the cabling runs accessible.
You know that classroom that isn't currently designated as a 'computer room' - it's now 5 years later with a different school head and they want it to be one.
Even if you don't run 20-30 cables per room (that would be very expensive, I know) make sure there are at least 5. If your school has money, which it seems to do (atm at least), then at least do that now. There will be possibility of a phone, PA/Fire Alarm system (now coming to you over VoIP), teacher's laptop, teacher's permanent desktop, interactive whiteboard, projector, network printer and any other of an array of goodies that could be fitted in to that room.
People tend to focus on the visible stuff in a room ie the computers. It is essential that you keep in mind that all of those computers require infrastructure to keep them operational. And keep reminding the people with the purse strings of that. It should become second nature to all of you every time you want a room with computers in it to then think to yourselves "OK, now how about the cabling - how much network cable will we need and where is it going to run to. How much power is this room going to need?"
And that brings me to another point. A classroom isn't like an office building. Electricians tend to plan things out as if it is an office. They can overlook the fact that in a classroom environment *all* of those computers could be booting up at once. Is that going to trip a circuit breaker? Remind them that it is a school, not an office where people roll in one after the other and boot things and do things a few at a time. Also if you are putting in heat/cooling stick it on a different circuit - assume that everything will be running all at once at full capacity even if only for 15 minutes at a time. Also keep thing in mind for future proofing the place - even if wiring it all now will be prohibitively expensive make sure that they leave enough leeway and access to allow for more future expansion.
I shouldn't have to point out now that similar principles should be applied to the networking and server side of things in the back end? All of those cables will need to plug in to something somewhere along the way. Right now it is safe to have a 100mbit connection to each computer (unless they are video editing and saving Gbs of data to the network) - but for the love of god please ensure that your core network is gigabit and redundant. A few classrooms of students all streaming video is going to bring the entire network to a grinding halt if it isn't.