Dorchester native Mark Wahlberg revealed on “Late Show with David Letterman’’ on Monday night that he’s going back to high school online so that he can finally graduate. Wahlberg — whose Boston-based movie “Ted’’ comes out June 29 — attended Copley Square High School, now called the Snowden International School at Copley, but dropped out in 9th grade. He is now the celebrity ambassador of The Taco Bell Foundation for Teens Graduate to Go program. Wahlberg told Letterman that Massachusetts will honor the credits he managed to earn before dropping out (“probably like one or two,’’ he joked), and that he’ll take classes while waiting around on movie sets. Wahlberg was spotted over the weekend at the MannyPacquiao vs. Timothy Bradley fight at the MGM Grand Garden Arena in Las Vegas.