They can involuntarily force you to leave, but, usually, what they do, according to all of these students, is that they kind of force you to take a voluntary withdrawal, and that's what puts you in the process.
Harry, Ron, and Hermione talked more fully and freely than they had all week as the train sped them southward. Harry felt as though Dumbledore's speech at the Leaving Feast had unblocked him, somehow.
Mrs. Weasley was intrigued by the Whomping Willow, which had been planted after she had left school, and reminisced at length about the gamekeeper before Hagrid, a man called Ogg.
Stojilkovic hated school and, after leaving at 16 with no qualifications, she got a job at the high court, where she served lunch to the judges and jury.