Jess usually experiences Mrs. Myers as an eagle-eyed person who can tell when he's not paying attention and tries to keep him in line. He realizes she's more than that, though, when he returns to school after the tragedy and she calls him out into the hallway. He thinks she's going to tell him off and is judging her as mean and uncaring, but is in for a rude surprise when he sees that she does care. She says she wants to "offer him sympathy" and explains how sad she was when her husband died. This humanizes her to Jess, who'd never thought about his teacher's personal life.
Maybe her grief about her husband explains her short temper or strictness with the other students; certainly she's one of the few people in Lark Creek who saw Leslie's value and potential, although not as deeply or in the same way as Jess did. Of all the adults who interact with Jess after Leslie's death, his teacher is one of the ones who helps him the most: "Mrs. Myers had helped him already by understanding that he would never forget Leslie" . He was wrong to judge her so harshly. She's really a good person, who cared for Leslie and who cares for him.