School Ghosts typically appear only by force, either when (depending on the level of school, ie; college, high school etc) they are either a: caught and fined for truancy and forced to and attend class for a few weeks to please the courts.
Or b; periodically go on 2 to 4 week "benders" and are spazzed out of their minds on booze and drugs until they remember that they have class and pop in for a session or two. Just enough to flunk a test or midterm and copy someone Else's lecture notes.
"School Ghosts" normally are borne of those who become tired of actually getting up in the morning to attend classes, homework, studying or exams. These entities names are typically called on role or roster every class session but are rarely accounted for. SG's often appear in the back row, with dark sweats or flat out pajamas no supplies and fall asleep half way through and aren't seen after that.