Based on another question, squib (and intuitively, muggles) won't be accepted into Hogwarts.

Admittance to Hogwarts is controlled by the Quill of Acceptance and the Book of Admittance. You name gets written in the book if you demonstrate signs of magical ability while you’re young, but this has to be your own magic: it’s not enough to ride off the coattails of your parents.

Squib have some background radiation magic around them that eventually wears off so they cat get in.

But what about the low aptitude students, students that fail to pass classes or graduate to the next grade. Do they get kicked out? Held back? Special school? Summer school?

(I'm american, not familiar with English boarding school practices either).