For unknown reasons, Mr. Tedesco had Mallory as his student a year before he had Alex despite Alex being a year older than her. Like most teachers that had Mallory, he did not look at her kindly due to her poor academic performance. Though the feeling was mutual due to Mr. Tedesco's rude and obnoxious personality.
A year later, Alex was Mr. Tedesco's student and figured out that his cousin was on the faculty at Dartmouth, with Alex realizing that if he could get a letter of recommendation from him he was sure to get in. When his family was heading to Parent's Night, Alex urged his parents to win Mr. Tedesco over and so Elyse went to meet with him. As soon as Mr. Tedesco managed to insult a bunch of mothers and their kids, Elyse demanded he apologize and after he refused she punched him in the face knocking him out.
The next day, Steven went to Mr. Tedesco and apologized for Elyse's behavior and earned Mr. Tedesco's respect. Unfortunately, he managed to go off insulting Elyse further and also asked how she was in the bedroom prompting Steven to punch him in the face in the same way.
Later, Mr. Tedesco was invited to dinner at the Keaton House as one final way to apologize for everything. Despite him continuing to be insulting to everyone, the family put up with him and apologized, even deciding to heat up some frozen steaks due to the fact that he said he had already had chicken for lunch and wouldn't eat what they were cooking. Eventually when Alex was alone, he told him that he had earned his respect and give him the letter of recommendation. Though after he suggested that the rest of the family was less evolved than Alex, Alex decided that enough was enough and tore up the letter of recommendation finally standing up for the family. Steven then told Mr. Tedesco that they thought he should leave and opened the door and Mr. Tedesco let himself out.
A minute later, he knocked on the door deciding that he would eat the chicken, but Steven then slammed the door in his face.
Mr. Tedesco can be summed up as an overtly rude and obnoxious individual who constantly thinks of himself as being superior to everyone around him. When talking with or discussing anyone, he typically throws in a few insults about the person he's talking to or someone that that person is related to. He has a tendency to be rude and unfair to his students, giving them excessive amounts of homework and giving them cruel nicknames such as "Larry the Loser". To earn his respect a student has to work hard and do well, and so Alex was able to earn Mr. Tedesco's respect easily. Whenever talking to a group, he only answers questions that he considers intelligent and will leave prematurely if anyone demands that he apologize. His obnoxiousness is so outspoken that it can even cause lifetime pacifists like Steven and Elise Keaton to punch him in the face, even if they feel very bad afterwards.
Getting punched by someone will not change his mind about them, but he can be persuaded to forgive them if someone close to them apologizes sincerely. Unfortunately this does not help him much as his rude mouth tends to get him punched again even after all has been forgiven.
It seemed after he was punched by Elise and then Steven that he became a bit frightened of them until they offered their apologies.