Your son's behaviour - implied by the teacher's comments - sounds like my son's. He has trouble concentrating and needs help to stay focused. We've been working on it with him, and school have too, but it remains work in progress.
My son has been on report a few times.
Although I wish that his behaviour hadn't warranted it, I think that being on report has actually helped him for a number of reasons, and his concentration and behaviour have improved as a result.
First of all, as your son's form teacher has suggested, it gives the opportunity for him to be rewarded for being good rather than criticised/punished for being disruptive - that helps reinforce good behaviour. It's nice being praised.
Secondly, it breaks the 'behaving well all day' into bite-size chunks - he has a one-hour goal of behaving well in each lesson, and just has to concentrate on that one goal at a time. Most children are able to rationalise that themselves without the help of a report card. Some don't naturally think like that, so need to be prompted to do so.
Thirdly, at least the way they do it at our school, they set particular things that he needs to do to get ticks on the report card - so for children who need reminding of what behaving well means (and I've no idea why my boy needs reminding - we've reminded him often enough - but some people just need more reminders than others I guess) they have three particular things to work on, which can seem a bit more manageable.
Fourthly, and this is something you might want to get your son to focus on, if he's being good in most lessons but just not in the form teacher's art class, the fact that he's got lots of ticks from his other classes will help the form teacher to see that it is not a universal problem and it may help show that there is a correlation, as you say, between lessons he finds difficult and his behaviour.
So, being on report is not always a bad thing in terms of the outcome for the child, and it's good to get things back on track early on in their secondary career.
And being 'singled out' by her wanting to ensure she sees you at parents' evening may not mean that she thinks he has the worst behaviour problems in her form group, but might mean that she thinks that, unlike the 'naughty' children, he has a soluble problem and she wants to work with you to make sure home and school are working together to solve the problem and have a shared understanding of what needs to be done. She might even be wondering about dyslexia and wanting to talk to you about having him properly assessed - I find that relying on a 12yo's account of what a teacher wants/has said/meant is not always reliable.
Good luck next week. It's demoralising hearing that your child is being disruptive - the combination of feelings of guilt and wanting to defend your wean is quite hard to deal with - but don't go into it assuming that the teacher is going to be unfair or prejudiced. Of course she might be, but most teachers aren't.