My SIL has just rung to ask my advice as a teacher but her question is beyond my experience so I’m asking here instead. She knows this!
My DN starts primary school shortly, In reception and the family has had some struggles over the summer (grandparent diagnosed with terminal illness, elder step-sibling significant mental health deterioration) which have seen behavioural changes and toileting regressions in the youngest.
My SIL wanted to know if I thought she should let the new school know in advance.
I suggested she sent an email outlining the basics and explained she would send in a change of clothes etc even though dn might be totally fine at school but forewarned is forearmed for her teacher rather than telling them on day 1.
Not sure this is the right answer!