Getting school to co-operate with child's mental health needs

My son (Y6) has been having a lot of problems with school and our GP wanted to explore autism as a possibility. He's had his assessment and it was a borderline decision not to diagnose him. Instead they said the focus should be on his mental health and they made loads of really good suggestions that school can implement. The problem I have now is I very much feel his school is trying to slow time dealing with this because he is leaving in July. I have emailed them twice asking for a meeting to discuss the report and all three of the people I sent the email to have ignored both emails. I haven't even had a one line acknowledgement.

Conversely our GP has been excellent and referred him to CAMHS. Also this week I copied his secondary school into the latest email and they replied within an hour - we were only notified of his school offer this week so I thought this was very responsive.

How can I get his current school to co-operate? His problems relate entirely to school, so there's not much we can do without their help. It's heartbreaking seeing him going to school each day looking so depressed. Do they have a legal obligation to act? I have started sowing a few seeds about him not being able to attend if nothing changes. They're obsessed with attendance and SATS, so I thought I could use those factors to get them to act.

Any advice on how to proceed would be appreciated.