You found the KEY. A single DEDICATED teacher can teach 50 students who WANT to be in that class and WANT to learn all they can and DO learn all they can.
A teacher TODAY cannot teach 15 students who DO NOT want to be there, have NO interest in learning, won’t study or do homework, pass their time texting or worse, if they even went to school today.
If their teacher pushes the issue, he/she is pounced upon by an administration and school board adhering to “political correctness” and fearful of losing a handful of votes.
Tell me about it. I teach public school in downtown Los Angeles.