'Your child is doing well at maths' - can't see a problem there. 'Your child needs to concentrate more in maths' - not exactly your fault. 'Your child is falling behind in maths' - response - what are you going to do about it and how can I support DC - again no issue.
'Your child is arriving late every morning' - well, yes, he could use that in court - but I can understand why he would want his child in school on time.
So, unless there are particular issue with your child that he could use, I still think 3 is not as likely as 1 and 2.
When you're there I would be as calm as you can be and have some stock phrases that you have worked out that you can use - like 'what resources can I use to help my child in the evening' etc - you hopefully will know what sort of comments might come back that you will need to deal with. Make sure that you don't have a row with exH - you can come out of this looking like the reasonable one. It is perfectly acceptable to say, in front of the teacher, to your ExH 'Please do not speak to me like that' - but I would be speaking to the teacher and ignoring him as much as possible. Don't be afraid to ask the teacher questions about things that have been moved on from because ExH has been the main talker.