@Linkat,
The problem with your comment is that the nurse may have been qualified to make that call, but for all we know the school may have implied to her that only extreme cases go to Emergency.
When I worked as a camp cook/medic, I was the only one to make the call for a helicopter, or if and when an injured person guy returned to work. However, every company made it clear I was NOT to call a helicopter ($$$$), but have the guy driven to hospital. Problem with that is no one provided the camp with a vehicle in which a supine person would fit. If there was a spinal issue, or badly damaged legs, the only way to transport is on a spine board... this is one of the reasons I quit - no support for their workers. I would have called a helicopter anyway but thankfully I didn't have to. The other thing is, we were often out in the hinterland and driving would have meant bumps and jarring - not good for a spinal or head injury and very painful for ribs, leg, and other injuries.
So, there may well be some politics going on.
In this case, I'd call the parents, describe the injuries, and ask if they wanted an ambulance or if they'd rather come and pick them up. If the parents are paying, it's really their call. The nurse could easily have asked the mother when she told her about the broken arm.