Whenever you walk into the wrong classroom

Once when I was new in school and I wasn't used to the timetable switching yet, I went in the wrong class only there was a substitute and he didn't know I wasn't supposed to be there and my name wasn't on the list, so he actually added my name to the class list. And then I realized that there was a girl laughing at me and that she wasn't in my class and then I realized that that wasn't my class so I told the teacher and I was late for my real class and when I came back next period, he smiled at me as if we had a secret and I was mortified.