He only needed one course to graduate which he took before Christmas break in Season 5. He didn't even have a job for most of Season 5. Dwayne literally evicted him in one episode for not paying rent and getting the lights cut off. And for the first three episodes of Season 6, he didn't even have a place to stay.

Ron Johnson's continued presence at Hillman during Seasons 5 and 6 of A Different World can be attributed to his delayed graduation and his eventual transition into a local business venture.
His timeline following his senior year includes:

Delayed Graduation: Ron did not graduate with Dwayne and Whitley in Season 4 because he failed his European history final. He officially graduated in January 1992 after completing a ninth semester, which explains his student status for the first half of Season 5.

Employment Struggles: After graduating, Ron faced significant instability. He worked various odd jobs, including a stint as a phone sex hotline spokesman and a car salesman. His financial difficulties were a major plot point in Season 5, leading to the rent and electricity issues you mentioned.

Business Ownership: Ron eventually found a permanent reason to stay in the Hillman area by becoming the co-owner and manager of "The Place Where The Blues Will Be Played" alongside Mr. Gaines. This established him as a local businessman rather than a student.

Personal Ties: By Season 6, Ron remained in the core group due to his deep personal connections, specifically his long-standing friendship with Dwayne and his evolving romantic relationship with Freddie Brooks.