Well, “expulsion” from school is generally related to activities during school or school-based extracurricular activities. I’m not sure what would be accomplished by having educrats jump through some administrative hoops to formally “expel” these students, as long as there’s no risk of their actually returning to a regular school. It’s not like the absence of an expulsion on their school transcripts is going to give them a leg up on future employment or college admission — failing to complete regular high school because you were away serving a prison term for murder is kind of a deal killer for any meaningful employment or post-secondary education.
“Hrmm... well I see here you did a few years for murder... hrmm... member of the debate club very good... wait! What’s this? You don’t have a diploma or GED? This interview is over!”