Generally speaking when schools won’t accept students, it is because those students do not abide by the school’s standards. It is not about learning full-time or not learning full-time, but about following the school’s standards in terms of tznius and shmiras haayin, etc. A school has a right to set standards. The school is responsible for its students and it is the school’s responsibility if the child ends up being “over” on big issurim as a result of attending the school. A parent has a right to choose a school for their child that lives up to his standards.
Unfortunately, people sometimes don’t want to admit that the school didn’t accept them because of their lack of tznius, so they will come up with other reasons, claiming that the school is prejudiced against them because of x,y, or z, without acknowledging that it is because of their refusal to abide by the school’s rules.
I recently met a lady who was insisting that the reason that none of the local schools accepted her kids was because she was divorced. From looking at her, I could think of plenty of other reasons. In the course of the conversation, it came out that they didn’t accept her kids because she doesn’t cover her hair (and since she doesn’t cover her hair because she is divorced, so to her, that translates as, “they won’t accept my kids because I am divorced.”)