If there was a well-trained teacher supervising the kids, and he left quickly because of an injury, then there is no moral fault with the teacher or the school, but factually it could be their fault because the second accident happened when no supervisor was present.
Technically, the school would be at fault in that situation as health and saftely rules are there to prevent injuries or accidents happening to someone who is responsible for looking after a room of students, and thus leaving the students unprotected.