You can usually recognize a Down’s child by features. In any even, the teacher should have been given a heads up.
I have an adult Down’s brother-in-law, and if he cursed, he likely wouldn’t even know what he was saying. Down’s people have different levels of functioning. The teacher goofed, but sub teaching is a TOUGH job, and I think the teacher should be given an opportunity somewhere else. Many Down’s children are poor candidates to be mainstreamed in regular classrooms.
If I thought a student was being belligerent, I’d send them to the principle’s office. In these litigious days, physically accosting a student can be very bad for one’s career.