Jaylon Mason expected Wednesday to be just another normal day at Dorchester’s Brooke High School. Then, at a seemingly mundane school assembly, Jaylen Brown walked in through the door. Stunned, the hundreds of high schoolers at the auditorium went berserk, screaming, jumping out of their seats, completely incapable of containing their excitement as a thrilling reality set in. One of their favorite athletes, one of their biggest role models, was at their school. “I saw a couple kids even cry,” Mason told CelticsBlog.