Mark Dobbins was a gym teacher and football coach of Chestnut Academy and a suspect in the rape of Holly Purcell.
Background
One night, Mark was drinking at a bar, depressed that his fiancé had left him. A teenage prostitute approached him, offering her services for $100 and lying about her age. Before Mark could refuse, an intoxicated police officer immediately arrested him. The charge was subsequently dismissed, and over the next ten years, Mark married a woman named Julie and had two daughters. After the football team he coached won the county championship, his picture and name were mentioned in the local paper. The cop who arrested him recognized him, called the school board, and got Mark dismissed on the spot as the state pulled his teaching license, while no other school would hire him because of the arrest record. As a result, he had to forge a document to get a teaching/coaching job at Chestnut Academy in Manhattan.
Contagious
Detectives Olivia Benson and Elliot Stabler interview Mark and Julie, who are friends of the Purcell family, regarding Holly who was reportedly raped. One of his lacrosse players, Kevin Wilcox, reveals to Benson and Stabler that Dobbins is the gym teacher and a substitute is filling in for him right then because he had a game that night, which gives the detectives a reason to suspect Dobbins. Holly then tearfully claims that the man who victimized her was Mark. Benson and Stabler arrest Dobbins in the middle of a football game. Later they interrogate him and find that his activities includes hosting sleepovers and a summer camp in The Poconos for girls, and teaching gym. Dobbins was among many who were interviewed, and when he reveals his assumption that Swift is dead, the detectives think they have further proof of his guilt. Stabler suggests that he killed Laura Swift and raped Holly and his own daughters, and he finds the notion so repulsive that he demands a lawyer for any further questioning.
News of the victimization of Holly gets around as well as that of Laura, and other parents come forward with their children that were allegedly molested, thus preventing Mark from making bail. As the detectives unearthed his records, Mark has a sealed arrest record from the Erie County District Attorney, which he explains to ADA Casey Novak, despite advice from his attorney Trevor Langan not to do so. Due to some circumstantial evidence by a teenage prostitute and a "drunken cop," Mark was falsely arrested (although the charges were subsequently dismissed), and revealed that he had to forge documents indicating he was allowed to work with children. Novak thinks she can use this to get a confession from him on the whereabouts of Swift despite Langan's protests, but he still adamantly denies raping or killing anybody, which prompts Novak to ask the judge to pose the maximum sentence during trial.
The rape charge against Mark was dismissed after Holly recanted her accusations. Although Mark's name was cleared, Mark was dismissed from Chestnut Academy, not because of the case, but for forging the document destroyed his integrity, which could be an endangerment for the students. It is implied that Mark is also permanently blacklisted from ever working again as a teacher or coach in any school system, whether it is public or private. Stabler apologizes for what has happened, in which Mark agrees, and he and Julie walked away. He and his wife are currently living with his in-laws in Upstate New York.