Schools have a duty of care to children.
This means if they cut themselves, vomit on themselves or soil themselves they have a duty to assist them out of significantly stained/soiled clothing.
Reception aged children have had issues with this since schools began.
When schools employed nursery nurses in reception there was no issue, but now there is no funding for this sort of help or staffing.
This leaves schools in an impossible position.
They have an obligation to help, but do not have the staffing to do so. There is also the issue of safeguarding a child/maintaining their privacy and dignity.
Some encourage children to change themselves, others ask parents to come in (which they can refuse to do).
I feel so sorry for the schools, their obligations are massive, their resources are tiny. There is no perfect solution.