We never saw an episode celebrating either of their graduations. JJ had to repeat the 12th grade twice and then started working at the chicken shack.

Thelma was supposed to become a professional dancer/singer/surgeon and fumbled multiple bags along the way only to end up with drunk, alcoholic, abusive Keith.

Michael just popped up one day and started going to some night classes? JJ and Michael were still sleeping in the same bed by the end of the series.

That is a good point and and made me curious too. It seem JJ already graduated from high school in the 3th season and was taking Art classes. But it seem Thelma was still in high school in early part of 3th season ("A Loss of Confidence"), and already graduated at end of 3th season ("The Breakup") were she was pondering whether move to LA with boyfriend Larry or not.

JJ had to repeat the 12th grade twice and then started working at the chicken shack.

JJ didn't have to and didn't repeat the 12th grade ("Junior the Senior"). But James had the funniest line in that episode where Florida was asking JJ some history class questions. James was looking at JJ's report card and listening to his answers and said:" I am looking at grades Cs, but I am hearing F" :D

While the show didn't air formal graduation ceremonies for the siblings, their educational and professional trajectories were addressed by the series finale in 1979:

JJ Evans: He did eventually graduate from high school, though his path was non-traditional. After initially being held back and working at the Chicken Shack, he matured and found professional success. By the series finale, he achieved his dream of becoming a professional artist, selling a comic book character called "Dyno-Woman" (modeled after Thelma) to a national syndicate.

Thelma Evans: She attended both high school and community college during the series. While her career aspirations shifted, her "happy ending" in the finale focused on her personal life: her husband Keith recovered from his injury to sign a professional football contract with the Chicago Bears, and she announced she was pregnant.

Michael Evans: He was consistently portrayed as the most academically focused sibling. In the final season, he was attending college and, in the series finale, finally moved out of the family apartment to live in an on-campus dormitory.

By the end of the series, the entire family finally moved out of the projects. JJ moved into a "pad" with female friends, while Michael moved to his college dorm.