Continuity mistake: When Jack is in his new classroom for the first time, he sits at a metal desk. He falls with the desk when he tries to reach his book bag, which is sitting next to a broken wooden desk. Next, he sits in the same - miraculously whole - wooden desk, then proceeds to break it to smithereens.
Continuity mistake: Jack is at home after he had the heart attack (or whatever it was). His mom comes in to his room to tell him he can't go to school anymore. Jack gets upset and rips the heart monitor thingies off his chest (notice his shirt is unbuttoned). The camera cuts to his mom for about a split second, then back to Jack. Now his shirt is buttoned. There is no way he would have had time to button it.
Continuity mistake: When Jack is playing basketball with his friends, he has on a pink shirt. But afterward, when he is lying to the kids' mother about being a principal, he is wearing a red shirt. He is not in shadow.
Other mistake: During the graduation scene, as Jack and the boys leave in the car, there is a banner in the background (I think it may have been used when the movie moved on seven years) which says 'seven years later'. Surely it wouldn't be up on a graduation banner?
Continuity mistake: There is a playground scene where Jack (wearing a pink shirt) is talking with a school mate. His shirt is unbuttoned. The next shot is of the mate and when the scene cuts back to Jack, the shirt is buttoned up.
Continuity mistake: When Jack is playing on the concrete 'balance beam' (Jack is walking on it like he would a balance beam) at recess on his first day of school, his shoes go from untied, to being tied, and then back to being united.
Continuity mistake: On Jack's first day of school in the playground the top button of his blue plaid shirt is buttoned in some scenes and unbuttoned in others.
Visible crew/equipment: When Jack is begging his dad to let him stay in school, one of the double doors is closed and the other is open. You can see the reflection of someone moving in the door handle of the closed door.