Could this be separation anxiety rather than anxiety about school?
If so there are lots of ideas here: https://childmind.org/guide/quick-guide-to-separation-anxiety-disorder/
Being able to keep photo or reminder if the parent or call or receive messages and check the parent is OK during the day can help.
I have seen this in children with undiagnosed autism, and it can also, very rarely, be a childhood indicator of bipolar.
If it’s more school based anxiety, anxiety resources such as “The Anxiety Gremlin” and “what to do when you worry too much” may be useful, but really CAMHS involvement & CBT the ideal approach. Though hard to arrange!