What did the teacher do with ogar's cheese report worksheet key?

He grated it.
The word grate has the meaning of rubbing a certain objects and transform it into smaller pieces.
The word has a very similar sounding with 'grade' which is an action taken by the teacher to measure the performance of their students.

The teacher thought that Ogar's cheese report worksheet key was much prettier than her own. She was amazed that her intern was so creative and precise at the same time. She wondered in Ogar had greater potential as an artist or designer than as a poorly paid teacher. She replaced her own worksheet key with Ogar's.

White dwarf is very hot, but also very small and that is why it may appear dim.