Hi Mohammed,
For sanity's sake, could you trace your actions when maintaining students with user TESTGPC (use transaction STAUTHTRACE).
I would just like to verify whether we are seeing a structural authorization error or whether it's simply 'standard' authorization that's messing with us here.
Also, when you're maintaining the student in your example above and the error message turns up. Is this student's object ID visible in the authorization view at all? (report RHAUTH00 or in t-code HRAUTH).