I've had experience with this.
My daughter isalways bruised. I wasn't worried about it at all as she was very active and I bruise easily.
The school did start asking questions though, and one day my daughter told me the head teacher took her aside to examine a particularly bad bruise, we were mortified.
We went to the doctor, just to cover our backs really to be show we were not trying to hide anything.
It turned out she has a bleeding disorder which causes her to have an extremely low platelet count, called ITP, which causes hers.
That's pretty rare so not likely to be the cause in your case, but what I learnt from that is that you should not be embarrassed to talk to the teachers about bruising your child has, and how they got them. Be really open and upfront about it, to help them with their safe guarding.
They are looking for bruising in unusual places. Fronts of legs and bony places are typical. My daughter very often had a bruise on her temple though, and her bruises go a very deep purple colour, in hid sight they do look different from 'normal' bruising, so no wonder the teachers were concerned.