As a school you'll have to weigh certain costs:
The cost of staying where you are VS where you want to go.
The cost of updating ram vs not.
The cost of ram to get a bit more life out of the computer vs getting
new computers.

Ram for the older machines is horrifically expensive. But, think about
this: I have looked at several times this past year setting up thin
clients for various schools/club/etc and it pretty much comes back to
this: To set up a "lab" of any type you need one really good computer.
It needs to be a decent work horse of a machine. It should have a good
amount of ram.. like 2GB or more. The cost of a single new machine vs
updating all the old ones... there variables there I wont get into here
that need to be addressed.

I read a lot of papers about people doing it on less (ram) but no one
showed me it worked very well. And in the end, if the end machines, the
Clients, are so anemic of ram, it might not help you much to do so anyway.

25 clients can run from a single machine but you're going to need a
pretty powerful Head computer.