Sit, Not Squat

There are phrases that I say at least 10 times a day. This has now been added to the usual. Brandon has developed a new habit of squatting on his seat instead of sitting. It's not just at home either. The teacher mentioned it when I saw her last. Not sure why he's doing it. This morning it was especially bad. Not 5 seconds after I got him back on his butt, I'd notice him switch right back. Any ideas??????