How do cancelled classes work in high school?

Tommorow two of my classes have been cancelled so school is only from 2 to 4. Sometimes when a class cancels in the middle of the day you’ll have a free hour where you can do whatever (including going home, which I do when i get two free hours in a row). Cancelled classes (and planned free hours) are very frequent I get them almost every week. I wonder how cancelled classes work in the US, given that it doesn’t seem to be the norm to leave during school hours. Respond with how your school did it if there is no consensus about it in the US