Can you please explain this joke?

[Axeloblivion]

Heres the joke, I just don't get it:
On the first day of School.
A student died. Who killed the student?

Choices :
English teacher says ' i was having karaoke in another section
Science teacher says ' i was dying my hair in the office
Math teacher says ' i was resting from computing the grades
P.E. teacher says ' my students and i were thumb wrestling

Guard says ' i waz having a dance battle with the other guards

Who was the killer?

[ThirdEarthAge]

This question has been asked so many time it makes my head hurt. If you have the answer please give it. Most people want to say math teacher but yes you can grade papers on the first day so that can't be it.

[mermee]

math teacher. How can you compute grades when its the first day.