Mystery puzzle and riddles

It was the first day of the colloge when a young girl was found dead on the colloge premises.The police narrowed down four suspects

1.The dean: he claim he was in his office whole day.

1. math teacher:she claim that she was giving her student grades
2. Peon: he claim that he was bringing the mails
3. mathew: he claim he was cleaning the roof of the college

WHO WAS THE MURDERER????