I think you might be better off contracting somebody to do this work for you if you have to come in here asking such a vague question.

It depends completely on all sorts of information that you haven't said.

How many classrooms, for starters?
How many computers per classroom?
Where would you house any of this equipment?
How far would it be from that place to every other classroom that needs to have access?
Is anything already bought?
Is there any networking hardware already installed behind the walls?
How fast do you want this stuff to be?
Does it need to be available from outside the school or just inside?
What all do you need to be able to do on this intranet?
Are you sure you know what intranet means as compared to other terms like LAN?
What is the expertise of the people who will be managing this?
Do they only know how to use Windows, or do they also know Unix and Linux?
Do they know Apache or IIS? Or perhaps something else.
Do they know how to use SQL, Sybase, Oracle, or some other sort of databases?
Do they know Sharepoint?
What sort of budget are you prepared to sink into this effort?
Who are the major players and decision makers involved in this effort and do any of them have any particular requirements as to choice of solutions?
Do you have the tools necessary to install anything you might need?

All these questions and a whole lot more need to be answered before we can even begin to make any kind of serious suggestions for you.

As I mentioned before, I would suggest not trying to lay all this out for us. I would suggest you consult with skilled tradesmen in your local area that handle this sort of thing on a routine basis and who can come by and physically examine what you have now and how everything is laid out currently and all of that.

My fear for you is that we spend weeks going over all of this stuff trying to get you sorted out sight unseen and we miss a whole host of significant details purely because we haven't been able to access your facility and we just don't have the ability to visualize everything adequately.

It also helps to have people invested in your success. While many of us do genuinely care about the person on the other end of the thread, we aren't paid to do this and no harm comes to us if we fail to advise you adequately. It is quite a bit different to do this sort of thing professionally and contractually. Your expenses would increase, but your risk would also dramatically decrease.

I am willing to attempt to advise you through this whole process, but I can't emphasize strongly enough that it might not be a good idea to discuss this here in general.
 
If it's realy only for intranet (html pages), set up an old desktop with linux. There are several distributions that have everything pre-configured you need. You will be amazed what an old box can handle if it's only network traffic and no interactive use.