Elton is known to hold grudges for long periods of times as he was still upset about Tess rejecting him 20+ years ago. Even though Anna was innocent, he took his pain out on her as she was Tess' daughter not caring whether or not she did good on any of her assignments and failing her. He would even send her to detention for small reasons such as making a sly comment that he was bald behind his back. Although he seems to be much lighter to his other students as he gave a B on a pop quiz to a student who gave a very poor answer.
According to Tess, when he was in high school, he was weird.
Elton attended Griffin High School with Tess. He had a crush on her and asked her to the prom but she turned him down. Starting a grudge that would last well into his adult years. He eventually became a English teacher.
When Elton becomes Anna's English teacher, he does anything he can to torment her. He gives her F's on all her work despite good quality, sends her to detention when she tries to ask about her grade, and gets on her back when she talks to her love-interest, Jake. She talks about his treatment to her mother, but Tess (who is not aware that it is the same Bates she knew) doesn't believe that her English teacher is out to get her.
When Tess and Anna switch bodies, Tess attends Elton's class as Anna and notices that he looks familiar. He gives the class a pop quiz and gives a student who gave a poor answer a B but "Anna" who gave a detailed analysis a F. When "Anna" confronts him he claims that her performance was poor and claims what's done is done. At that moment Tess realizes who he is and why he is so mean to Anna. Tess scolds him for tormenting an innocent high school student over a petty grudge against her mom and tells him to let it go or she will inform the school board. She also reveals that the reason she said no to him was because she already had a boyfriend and he was weird. To which he gets a guilty reaction on his face.
What happened to Elton afterwards is unknown, but it's assumed that he stops being unfair to Anna, after she (while Tess was in her body) threatened to rat him out.