It's not just a USB driver though. You've got to support PCI (or something) to find the USB controller, then you've got to fiddle with that, then you have to discover the devices, then you have to interface with them.
Reading from a keyboard is much simpler. Probably makes more sense to run the computer headless with a serial terminal and make the students poke at a UART. PC standard UARTs are pretty easy, and I think it's easier to find a motherboard with a COM1 header than a PS/2 port (at least my latest hardware matches that description).
You've got to support PCI (or something) to find the USB controller
PCI is nothing in comparison to the complexity of USB, especially since the BIOS will usually have set up the I/O ranges for the devices already. It's literally a few dozen bytes of machine code to scan the PCI(e) bus for the device you want.
Oh sure, PCI isn't too bad, but it's way more work than inb(0x60) or inb(0x3f8). (although to be fair, you probably want to setup an interrupt, which is real work)
Reading from a keyboard is much simpler. Probably makes more sense to run the computer headless with a serial terminal and make the students poke at a UART. PC standard UARTs are pretty easy, and I think it's easier to find a motherboard with a COM1 header than a PS/2 port (at least my latest hardware matches that description).