'lazy' is this a word teachers should use in reports??

Should use? Probably not, but I agree that these reports need to be filled in quickly, with little time to either compose thoughts or be diplomatic. Some of my classes have four, five, six kids on report. I don't have. Whole minute to think about and write on each one - I'm too busy teaching!!!

I think you may be being over sensitive, but ask your son if he tried to follow the instructions and ask to see the work he completed during the lesson. That's good parenting. Not automatically being defensive over the use of a single word.

Is you son lazy?
Was the teacher being accurate?

"X needs to ensure he tries as hard as he can in every lesson"

"X does not always work to the best of his ability"

Both these phrases mean "X is being lazy" - it just takes longer to write.

I'm a teacher and I try not to use the word lazy when talking to students about their work or to parents. (I do use it when students are too lazy to do things like throw away wastepaper).

If I had a pound for every time I skirted around the subject of "effort" at parents' evening and a parent said, "Yes, he / she is lazy," I'd be retired and living in a mansion.

Students who have ADHD can also be lazy. In this particular instance, I'd speak to your son about this particular lesson and, if you feel it's necessary, then speak to the teacher too. Is this his regular teacher or was it a cover teacher? Does the teacher know your son's diagnosis and the specialist advice for how to give him instructions and support him in the classroom? In our school, like many others I expect, SEND information is often communicated very poorly so that classroom teachers don't know about diagnoses or recommended methods of support for individual students. Of course, this isn't right, but if this is the case then the teacher (if they're your son's regular teacher) might well be glad of an opportunity to talk to you about your son's needs.