my geography teacher hit my friend henry in a lesson, do i tell?

[Josh]

henry had a big bruise on his back the next day and everyone hates the teacher, i think it would be harsh to tell on him because he might get sacked

what do i do?
i was there and i watched it happen, he was talking to me and he came up behind him and hit him and told him to move seats,

he isnt exactly a jokey sort of person

he was seriouse about it
the above wasnt particularly clear,

henry and i were talking and the teacher (whos name im not mentioning) came up behind him and hit him and told him to move seats. i should also mention i go to a school where we pay a lot of money for the education we recieve