I would ask the school if they have a policy and what it is.
I know my older brother started getting them around 5. My mom never kept him out of school when he got them. She taught him at an early age about not sharing forks/ cups etc even before he got cold sores.
As long as she isn't sharing anything that touches her mouth and itsn't picking at the sore and washing her hands after eating or something that she had contact with her mouth then everything should be fine. The only way to pass them on is by direct contact with them.
I pick at mine and don't always wash my hands after words ( gross I know..) but I have never spread mine to anyone. I just make sure I don't kiss anyone on the lips or let anyone drink out of my glass or give the little ones bites off of my fork when I have a sore. I don't normally anyways but Im more conscious about it when I do have one.