My Daughter Is 10 And She Has Been Getting Spots

My daughter is 10 and she has been getting spots on her arms and legs only. They are not raised or itchy. They look likes hickeys or small red spots under the skin. She only gets them when she is at school. She didn’t get one all summer. She went to a hematologist and they ran every test they can think of and came up with nothing. Other then the purpura like marks she was very healthy. I am now trying to see both and allergist and a dermatologist. Is it possible to be a reaction to something at school, maybe a cleaning product they are using? I just don’t get it. My guts says allergy but the hematologist says allergy would cause only a raised rash. Hers u cannot feel only see. I am lost. Please advise.