Unlike others here, I think you can probably do it as you're not asking for microphones or
anything before the mixer. Lets say you have a budget of £600 You can buy Soundking
speakers or similar (Alto had a recommendation in another thread today) for around £450
or less. Stands will cost around £40 and cables under £20 if you can solder the
connectors yourself. That leaves you £90 for the mixer. You won't get A&H standard for
that kind of price but the bottom of the Yamaha MG range comes in about that price and
there are Soundcrafts even cheaper.
Of course, if you really want value for
money you need to look at used gear but I suspect that a school will insist on new.
JRP Music - Audio Mastering and Restoration.