Since its for school you’ll want it to be reliable and to work 100% of the time. I’d just get a big brand and not connect it to the internet if you don’t want your data collected.
Other than that you can try to block the telemetry at the DNS level by VPN to your home with a pihole instance or using a private DNS.
If you really don’t wanna use apple or google OS, then best bet is to buy the tablet for the hardware and try to flash a different OS. But then you’ll risk it not working very well or having app compatibility issues.