Strike Back Tenfold

There were only three more days to the exam. The test papers arrived at the school two days ago, and just yesterday, it was stolen. It was obvious that an insider had committed the crime. 

The keys to the safekeeping room were in the hands of each homeroom teacher. The principal and the dean also held on to a key each. 

For the past few days, the dean has been out of state, carrying out his research. As of now, was still away. The possibility of him committing the crime was minimal. 

The principal was an even more unlikely suspect. After suffering such humiliation, the future of his career as a principal was already in jeopardy. He wouldn't dare to invite even more scrutiny upon himself. 

After eliminating the principal and the dean, the only other suspects were the homeroom teachers of each class. There were eight classes in the senior grade. The first, second and third classes were top classes, and the remaining five were regular classes.