The Celtics had just unveiled another school computer, this time at Manchester’s McLaughlin Middle School, and Jared Sullinger stopped at the desk of each student to autograph a picture of himself. The Celtics’ 2008 championship trophy sat on a table at the front of the room, with the walls covered with Celtics-themed murals. A girl, perhaps moved by these hints of excellence, told Sullinger that her team had just completed a 19-0 season. The Celtics forward had to pause after hearing that one. “Can’t say the same about myself,” he said, forced to reflect on the C’s just-completed 25-57 season. But for Sullinger, the new season started last week during his exit interview with Celtics coach Brad Stevens. As Sullinger readily admits, Stevens went at him hard in the meeting, mapping out a work schedule for the summer designed to improve the forward’s conditioning. At this time last summer, Sullinger was recovering from back surgery, and hadn’t regained his fitness by the start of training camp. Sullinger’s conditioning then became a season-long issue. So Sullinger has a grueling summer with team fitness and strength coach Bryan Doo to contemplate. “It went well,” Sullinger said of his exit meeting. “Coach told me what he saw. Coach told me what I could do better, and coach was really honest. Now I know what he’s looking for. Sometimes you can’t say stuff during the season. You don’t want to change what’s going on during the season. But once the season was over he came at me and told me what he wants, what he needs from me. Now is the time to change.