Sell it with no warranty or repair agreement
1) if they are paying for it and it goes wrong what are you going to do when they come to you?
As above, offer a costed service to put Windows on it if they pay for windows, and again, sell the service with no warranty or repair agreement
2) if they get linux and can't use it and come to you what are you going to do?
PAT testing each one, can be covered in the cost of the machine
3) it will need to be electricaly tested before transfer.
Have a link on the desktop pointing to a user manual which can be written for less able users
4) if giving them linux how will you make it useful to them
Sell it with a VAT receipt and keep a copy, that way if it is dumped, you have a record of who purchased it, and the police/authorities can go after the buyers.
5) what happens if they dump it in a skip in 18 months and the waste management company finds your school details on it.