Mr. Do,
Anyway, on my test iMac I have found that if I change the permissions on the folder /Library/Audio from everyone (read) to everyone(write) the problem is resolved!
that is a very surprising solution.
I think, I misunderstood what your users wanted to do:
Our users are having problems dragging long loops >1 min into their project.
I thought, this meant that they were trying to add audio files to their project by dragging them to the time line and looping them. Your solution indicates that they are dragging not to the time line, but onto the loop browser to install the audio files as loops. Is that really necessary? Do they need to install such long audio files as loops in the library? Do they need that same file in many projects? If they only need it in one project, it might be preferable to drag the audio clip to the timeline and not to install it in the loop library.
But anyway, even if they really need to install loops in the loop library, GarageBand should be adding user Loops to their private ~/Library/Audio folder in the Home folder, not to the System Library: /Library/Audio
I find it very disconcerting, that you have to give read/write access to a central installation library to students, however trustworthy they might be 😀
Regards
Léonie