I had trained a student very early on, so I kind of expected this type of thing to keep happening. I hadn't looked into it very deeply.
Students are not trained - they drop the sane way homeless/immigrants do - ie. when you get a new citizen (which used to be either homeless or immigrant before students were introduced), now there is some chance (rough estimate: 20%) you get lucky to get the more valuable citizen (the student).
Training is a different event when those homeless/immigrants get upgraded (mostly through Job Centers).