Didn’t learn hilcos yichud in high school but is there a kid that doesn’t know the basics that a man and woman aren’t allowed to sit in a closed room together? Same way we learned about waiting 6 hours after meat without learning issur veheter.
People confuse heterim and don't realize the severity. For example, בעלה בעיר, would not apply in such a situation, but based on what they learned in fourth grade, they may think it's okay.
There are many examples where people either don't know or forget the basics. An example that comes to mind, is here in Israel, all cab drivers are male. A woman who needs a taxi to the airport for her 1:00 a.m. flight has a serious problem of Yichud, though most people don't even realize it.
(How many woman use male gynecologists? Does a woman know that she must go with her husband in such cases? Even a regular family doctor or a dentist, can be a problem if they lock the door)