I'm not sure this would be workable, you would need a USB card reader and
microSD card for each student (will your sansa take mSD?)
At the begining of class, Students plug their card and reader into a USB hub
attached to the teacher's computer.
Two of these for example
at the end of class the teacher copies from her recording device and pastes to
20 microSD cards. (perhaps this operation could be scripted)
Also I know you are not looking for players, just sharing.
$7 microSD card (have not used this I had the cards already)
$1 card reader