If you have a printer in your classroom it is very likely an HP printer. When alerting us to a need for a cartridge replacement, it speeds up the response time if we know the cartridge type.
The cartridge is model dependent, for example if you have an HP 1022n (marked on the front of the printer), then your printer uses a 12A cartridge.
To find out the cartridge your printer uses it is easiest to open your printer's door and look inside. If you feel uncomfortable opening up your printer, you can take down the model number and enter it here: