Found a couple of different citations. You do have to be enrolled and in good standing in a post graduate program

According to the NCAA, the rationale is: Current legislation requires that in order for a graduate assistant coach to be appointed, he or she must have either received his or her first baccalaureate degree or has exhausted athletics eligibility (whichever occurs later) within the previous seven years. It is now more likely that the graduate-assistant coach position will once again allow a recent graduate to be involved in coaching activities while continuing what should be the primary pursuit of the position -- a postgraduate education. This proposal is intended to establish two additional opportunities for individuals to advance their educational and career pursuits, while also possibly providing additional minority coaching opportunities.

Additional Info

Under NCAA rules, graduate assistants are paid an amount equal to a school's full grant-in-aid. Texas Tech, for instance, will pay its grad assistants $17,300. And the G.A.s must be full-time students in grad school.

As for coaching duties, grad assistants can be like full-time coaches, but they aren't permitted to contact or evaluate recruits off campus. G.A.s can phone prospects, though

the biggest problem facing programs now is the use of grad assistant programs  to bring in assistant coaches who are mid 50's and between jobs but have close ties to head coaches. The normal intent of the rule is evident for 20 somethings on the end of their career to continue their studies while getting into coaching. This new wrinkle is coaches taking care of friends while gaining a pretty good get for the coaching staff at a bargain basement price. Those coaches working on a staff while drawing a salary form a school where they have been dismissed and continuing to work in coaching is on the NCAA radar presently.