Just copy and pasting Ash’s letter from the aforementioned thread (certainly not in my capabilities to be this professional when it comes to child matters, too much emotion for me, so massive thanks to Ash):
Dear Mr/Ms Headteacher
Re: CHILD NAME AS HEADING
I have some concerns that my son xxxxx, has apparently been having private meetings with staff, which I have not been informed about. I believe you are aware that xxxxxxxx lives with both his parents, under a court order dated xxxxxxxxx (attached). As such both his Mother and I are "the parent with care" and should both be kept fully informed about matters relating to xxx child name. As part of court proceedings it has been established that there are no welfare issues relating to either parent's care of xxxxxxxxxxxxxx and the issue of the "lives with both parents" order verifies this.
If xxxxx is having any problems in school, please can you let me know. It may be that you are offering him some support if he is struggling with something, and if so perhaps you could let me know. I can by contacted on this email or by mobile phone on xxxxxxxxxxxxxxx. I look forward to hearing from you, or alternatively I could come in to see you to discuss the matter.
I also request that you do not discuss my email with xxxx's Mother and treat this matter in confidence as I do not wish xxxxx to be caught in the middle. His Mother and I unfortunately do not communicate easily so all information, newsletters, reports etc, will need to be sent to us both separately.
Yours sincerely,
xxxxxxxxxxxxx
For the email heading put Private and Confidential FAO Headteacher name
And send it to the school office email address