That's a great thought! Unfortunately, the cost is going to be extremely high because the XOs aren't made in small lots. Your options on getting some at this point in time would be to either make a large lot purchase from OLPC, or ebay individual ones until you've gathered enough.
After the economic equation comes the infrastructure one. Do they have electrical power available to charge the XOs and run the school server? Do they have internet connectivity to link the school server?
The technical challenge of installing maybe the simplest step, but maintaining a school server may require someone who's is not shy around technology there.