UNIX permissions only support one Owner, one Group, and Other (UGO). 
The 'Group' slot is already taken by the specific lecture group to give students write access.
If we use 'Other' to give the supervisor read access, every user on the system could read it, which would violate the requirements.
If we add the supervisor to the lecture group, they get write access, which also violates the requirements.

Because a file cannot have multiple groups or user-specific overrides under standard UNIX permissions, this cannot be solved.