I have a kid like this--have you ever had a physical eval? Has he ever been evaluated for PT needs or seen a physical therapist or physical medicine physician? I'm betting he's also clumsy. My kid has very mild CP (dx via neuro) and sits in all kinds of wacky ways due to weak core and joint hyperlaxity, which is common in kids with ADHD. If I were you I would dig a bit further, including ruling out any genetic issues. At a certain point kids who have masking no longer can and need to get documentation, which includes options for seating issues.

Feet on his seat, with knees up? So he is squatting?

Yeah can you ask more questions about why this is a problem? Maybe he’s falling or getting in other kids space or something and then you can work on that more specifically

There is no “properly”.

Let him just sit whichever way is comfortable FFS

What exactly is the issue? Why is sitting on his leg a no go for school? I can’t imagine what issue this would be causing. It sounds very controlling to want a child to sit a specific way in a chair when it is not bothering any one.

Might he have weaker core muscles?

It drives me crazy too, but the teacher has to accept that his body needs to do this. I’d let her know that if it keeps being an issues for her you will ask for a 504 that allows him to sit how he prefers. She needs to let it go.

Lots of teachers have special seats for kids that need it. Seems to help. My NT kids also like to sit on their feet or knees at dinner sometimes. Drives me batty but we make it through as long as they are not disruptive. It helps when my youngest agrees to sit on a riser pillow as a I think the grownup chair is a tad too low for her.

I would talk with the teacher. She may not like it, but that movement is what his body needs. She needs to accommodate that. Unfortunately, a lot of teachers find it distracting, or think it’s a behavioral issue, when it’s not.

Many people with neurodivergence are also hypermobile. Hypermobile bodies need to change position often.

I am also very uncomfortable sitting “properly.” When I’m sitting for more than a few minutes (and when it’s not grossly inappropriate to do so) I pull my legs up and sit with my legs crossed or with one foot under me. It just feels weird to sit in a chair with feet flat on the floor. I am also neurodivergent with severe ADHD so I’m not sure if it’s a sensory issue or a lack of core strength as PP mentioned.

Anyway, it’s not a huge deal in my life, but I wonder if physical therapy to increase core strength could help.

Does he need a cushion to elevate him? Sometimes kids sit on their feet to get a better arm position for writing.

Flopping around can be due to weak core strength.

I have a kid like this and there are probably a million other things I would prioritize over sitting correctly. Things like completing assignments, focusing in class, keeping track of his coat, etc.

If anything, instead of wasting mental energy on sitting correctly, I would use this as an opportunity to request yoga ball chair (or similar) to help with sensory processing.

Pick your battles.

DS is 8 and neurodiverse (has an anxiety diagnosis and a ASD "phenotype" No 504/IEP and presents NT). Since he was very little, he's had trouble with sitting properly. It drives me a little nutty at home and I'm always reminding him -- mostly unsuccessfully -- to sit on his butt at dinner (versus on his feet/with knees up) but I didn't think it was a big deal - he doesn't walk around or anything like that, just sort of flops at the table and/or sits on his feet. However, his teacher, who is a little old-school, raised it at parent teacher conferences as a challenge. Other than constantly reminding him, is there something I can do to help him sit on his bum properly in the right venues?