He said: "They had this new programme in Massachusetts where you can take the courses on line so I'm starting this month and getting my high school diploma.
"I've got to take on all the courses I've missed so I'm a little nervous.
"I'm going to make it happen though. I have a lot of downtime on movie sets in the trailer and stuff so I'm just going to blast through it in eight months or so hopefully."
Mark also revealed he had been in the running for a place in New Kids On The Block, his older brother Donnie's group, until he was sentenced to 45 days at Boston's Deer Island House of Correction aged 16 for beating a man.
Mark - who has children Ella, nine, Michael, six, Brendan, three, and two-year-old Grace with wife Rhea Durham - quipped to TV talk show host David Letterman: "I wind up going to jail and watching them on TV, they're making millions."
The 'Fighter' star added that after the group performed at his jail, he boasted to fellow inmates: "I was in that group", to which they replied: "Well you're the dumbest motherf*****r ever, cause now you're in here!"