Can you see if the disk really is full. Maybe there is an audio track that isn’t set to be visible that is trying to record? Outside of that you might need to have the tech folks at the school take a look.
it turned out it was a matter of resetting the project folder, which I thought wouldn’t matter it being mere MIDI…
Stranger still, when I later tried setting the project folder back to the original one - that I previously couldn’t record with - it then worked with the original folder!!