How do I unlock an old school iPad?

Devices that are supervised/configuration locked by an MDM platform can usually only be released from central management by the platform that manages the device. Be aware that Apple will not assist with removal of management profiles.

Unless you are lucky - and have the option to delete the profile from iPad Settings - you’ll need to seek assistance directly from the System Administrator who manages the device. 

Install or remove configuration profiles on iPad - Apple Support

How long is a long time ago? If it's more than 10 years then it's possible it may not have been managed using the school's MDM platform? MDM (Mobile Device Management) take up only really started happening in a major way from around 2016 onwards in my experience.

Have you tried resetting it using the instructions here?

If it's managed by the school, only the school can remove the management for use.

If its a supervised device, you might have to reach out to your Admin from School. If its in ABM/ASM, you can ask them to release it or if its an MDM managed device, ask them to release it. If not you can give a try by connecting it to Apple Configurator which is based on your luck.