Crime at EMS: Can You Solve It?

The story:

On the first day of school, the history teacher was murdered. The police had four suspects, the gardener, the math teacher, the coach, and the principle. They all had alibis:

The gardener was cutting the bushes.

The math teacher was holding a midterm test.

The coach was teaching basketball.

The principle was doing paperwork in his office.

The killer was arrested immediately. Who killed the history teacher, and how did the police solve the mystery?

Go to this for the answer https://docs.google.com/document/d/1RmnTJdQ_Z6ePRoQdcDLC-f5XKpCRifpKV5YMpQ3WFQo/edit?usp=sharing