The professor could implement these boring lower-level pieces in a library which fires couple callbacks, one on connect/disconnect to provide two descriptors for HID device and their reports, another one for handling the HID report packets.
BTW, USB specifies a small subset of HID interface called “boot interface”. It’s normally consumed by UEFI firmware who needs mouse and keyboard for the setup GUI which runs before OS launches. The subset only supports basic keyboards and mice.
Then it isn't write your own OS from scratch. The constraints of the library will force your implementation around it, even if you think you have a better idea.
I think for the USB in that OS, a C API with intentionally very narrow scope would be OK for the job.
The main reason why the real-life code is so complex is that scope being very wide. We have USB 2 and 3, mass storage, two-way audio, cameras and GPUs, hubs and composite devices, numerous wireless protocols on top, OTG, power delivery, power saving features, and more.
Sure, but even one assumption limits innovation. Even if the innovation is stupid, learning the hard way is useful, so I want at least one student every few years to try that stupid thing and then from personal experience tell the others why it didn't work out.
If we were talking about a serious project there are a lot of best practices that should be rarely (and then only carefully) questioned, but for something intended to be thrown away at the end of the semester challenging best practices is a great way to learn.
That said, PS/2 is a simplifying assumption as well.
BTW, USB specifies a small subset of HID interface called “boot interface”. It’s normally consumed by UEFI firmware who needs mouse and keyboard for the setup GUI which runs before OS launches. The subset only supports basic keyboards and mice.