Murder on the Orient Express
Who was the murderer? All of them did it.
No witnesses, no murder weapons, and no evidence.
The Math teacher gave him an assignment to solve all the Millennium Problems within a day; and the students head hurt like hell.
The English teacher gave him an assignment to write all the works of William Shakespeare within a day; and the students fingers hurt like hell.
The Science teacher gave him an assignment to make a Huge Mars Rover Replica within a day; and the students hands hurt like hell.
The PE teacher told him to do 1000 pushups & lift 300 kgs & reduce his weight by 50% while doing all the other assignments; and the students body hurt like hell.
Then they all went on to their alibis.
The student went about with the assignments and, sure enough, collapsed within 30 minutes with fatal physical and mental trauma.
I repeat : No witnesses, no murder weapons, and no evidence.
Why? That is already answered above, but if you mean why they wanted to murder the student, then that was because they were teachers and they wanted to teach him a lesson about life (& unfortunately, about Death)