Don't care. The parents are helping nobody with that line of thought. Move the child where A. he isn't a danger and B. where he isn't negatively impacting the 20 other students.
Simple. Just need the guts to do it.
Unfortunately school systems today (driven by federal and state rules) are focused on mainstreaming students with "disabilities" into regular schools.
Back when I was a K-12 student they placed these students in their own special school with specialized staff who could meet their needs, and where they wouldn't continuously disrupt a class of 20 students who want to learn.
What you propose goes against the entire modern educational philosophy to "mainstream" these "special" students.