As usual, the Sun has spun this story for maximum Hate Bate effect, but if you check the last line, it is a telling one.
This child could have stayed on the trip, her mother elected to collect her, she was not 'sent home'.
Schools are buttoned up to the eyeballs with H & S regulations which if contravened can lead to individual legal responsibility for staff and governors.
So they enforce rules, which have been advised to parents, to the letter, even if they appear trivial to outside eyes.
I think you will find here is a lot more to this story than the Sun has bothered to explore - why let the whole truth get in the way of a national teacher-bate - just when they break up for those 'three month holidays'?