I kept going to the wrong class for half a year
Which class what that?
It was a math class. Right subject, but wrong room.
Make sure you are going to the right class from now on. It shouldn't be hard to not go to the wrong class.
I kept making a same error and I even have a note on the timetable but then I didn't read it.
Make sure you read it next time. It's good to keep track of all your classes and correct rooms.