"I doubt it is a bowel/medical problem as she is usually very well at school"
My kids are usually very well at school, but have an underlying genetic condition that you can't see. I could've written this post in reverse.
My dd is frequently sick, so this complaint happened at her previous school:
"a girl in my son's class was sick all over the pavement outside school yesterday afternoon and today she's back in, I can't believe they let her, now the whole class will get ill. I've told him not to go near her"
She had a reflux attack caused by what she ate at lunch, no bug or anything and nothing that another parent can see, but that parent not only chose to judge me and my dd, but even told her child not to go near her- and all her delightful mummy friends and their kids too. My daughter was devastated.
At her new school she was ill with a migraine a few weeks ago (first time at the new school) and was utterly terrified about going back because she thought she was going to be shunned again. Thank God that didn't happen and the first person that greeted her in the playground was over the moon to see her and concerned about her.
Feel free to call the school to double check that they're applying the 48hr rule correctly, but please don't accuse this specific family, you have no idea what might be happening.