They quit, giving you some excuse. If they fail to achieve a good result they won't get into university so they will get a full time job and quit your casual job after less than a year. They could also be in the first year of one of the high failure courses like Law or Medicine. Law has a 60 percent drop-out rate after first year. If they fail they will also get a full time job.
Final year is worse.
When they finish their courses, they'll either nick off to Europe or look for a more challenging full time position. This sort of turnover is predictable and avoidable, if you know what to look for and what questions to ask in an interview. The natural tendency is to concentrate on a whole series of questions to establish if they are what you want. Concentrate on their needs first. It's amazing how many people scrub themselves out of a job at this point.