Plot hole?

Remember, he was chatting with a dead person. She probably told him.

"I'd figure he would just see a bunch of ghosts walking around with their necks at weird angles."
Or, and this is an extremely wild guess, the MIXED-RACE FAMILY HANGING FROM NOOSES that we see as well???

Obviously, Cole would've seen these victims all the time if he was in the gym doing sports, especially that family. Must've been very disturbing to him.

I wouldn't count on Millsey to evaluate a plot hole. So far, at least of the one's I've seen him claim as plot holes, his accuracy is 0.0%. He doesn't understand the meaning of the term.

A ghost told him

It's not a plot hole: He's seen the trio of hanged Puritan ghosts from the olden days when he was with Dr. Malcolm Crowe in the school hallway. He told Dr. Malcolm Crowe about what happens when the ghosts are near him or get angry; the temperature drops to below freezing and the prickly hair at the back of your neck stands up. He tries to show the hanged ghosts to Malcolm, but he knows that it's futile since Malcolm can't see them because ghosts can only see what they want to see. Those three ghosts, a blonde woman obviously tried as a witch, a quaker and a black teenager/quaker are seen in the school hallway right up the stairs they are Puritans who were tried and killed for daring to be themselves like something out of The Scarlet Letter and The Crucible. Not too many people know those were based on a true story; but anyway those three hanged Puritans were the victims of hanged execution around the time that Cole and the teacher were arguing about. That's what Cole was referring to when he said "They used to hang people here." It's unclear whether the three have talked to him and told him or if he's just seen them at a distance and put two and two together.