Under certain circumstances someone can switch their student visa to a work visa at the end of their studies, but I guess a large number of students won't be eligible to do this.
The rest of the students are working because they are here doing an NVQ and it's part of their course, working the number of hours they are allowed to (20 hours/week term time, 40+ hours/week during holidays) or they are working illegally (more than 20 hours/week during term time).