We had a WelshX who had 3 witnessed falls in about 14 months, 2 when ridden. Vet found nothing more than a sore back, recommended physio who noted he had a dropped right hip, almost certainly from a previous fall, and was unable to put weight through the joint. He was never lame ridden but never hooned around in the field with the others.
Re:the school surface-there's a big difference between a slip or a trip or a fall. Dodgy surfaces can cause a few of the first 2 but v.unlikely to repeatedly cause
falls. Horses are generally quite good at finding their 5th leg when tripping.