"It's not common, but it's happened plenty of times over the centuries.
In fact, a big part of why student access to the Imperial Reserve is restricted is the most notorious cause for spontaneous departure: Satyric abduction (though the Satyrs themselves would say it was Satyric fellowship). Roughly every twenty or thirty years (most recently in 1644) an older student will fall in with one of the great Satyric revels and adopt the lifestyle - sometimes they're encountered again years later, in many cases as powerful wizards in their own right, and sometimes they just vanish into the wild. Somehow, the Satyrs and/or something else in the Reserve tend to block any Astrological attempts to track the kids down.
Other students have cracked under the pressure of their studies, usually compounded by hostile relationships with their Regents, and still others have stumbled into magical opportunities or disasters that didn't allow for bureaucratic niceties.
One rather famous example of the latter: in 1483, a Hedi student named Conselo Jax discovered a homeless woman who was actually the partially Mastered Countess of Breuve; he freed her from her state on a Monday night and ten days later was the flag-carrier on her campaign to retake her homeland, his face hidden behind a mask of rubies and alabaster. The school had no idea where Conselo was until about six months later, when it came out that he had been adopted as the countess's heir, had changed his name to Selus de Breuve, and had taken up residence at the Floating Court on the river Bres."
GC and Schwarzbart, I just sent yours- I missed them the first time.