I can confirm that 'stop out' has exactly the sense described by Dragon in #1: to take a deliberate, temporary leave of absence from a university degree program. In the US it's often for a year, for example, to travel abroad or to do volunteer work. I seem to recall a thread on this very subject, though I won't try to search for it now. Not all universities allow it, but mine used to.
'Stop out' would be less appropriate for an unplanned or unwanted interruption, such as military service, pregnancy, or lack of funds. In that case I think we would probably just use some other paraphrase, perhaps even 'to have to interrupt one's studies' (though that's not really a fixed term).
'Drop out' is indeed usually final, giving up on school altogether. When people calculate educational statistics, they list the dropout rate, that is, the number of people who attend for a while but don't receive degrees, whether in high school or in college.
However, for me it wouldn't be unthinkable to say, e.g., 'She had to drop out of college and work for a couple of semesters to save enough money to finish her degree.' So it may be that 'drop out' would also cover the involuntary temporary interruptions; that is, the cases where people leave school not knowing for sure whether they will be able to return.