Hi Jan,
If you suspect it is authorizations, could you have your students try running an SU53 in the backend to see if it is throwing an authorization problem?
Or do an RSTT trace with one of the students (see http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/d0e3c552-e419-3010-1298-b32e6210b58d?QuickLink=index&… page 59) to determine the cause?