He threw the ball straight up and it hit his face when it came back down

On the first day of the school year a young girl was found murdered. Police suspect four male teachers and question them. They were asked what they were doing at 8:00 am.

Mr Walter: "I was driving to the school and I was late."

Mr Thomas: "I was checking English test papers."

Mr Benjamin: "I was reading the newspaper."

Mr Calvin: "I was with my wife in my office."

The police arrested the killer.

How did the police find the murderer?