My high school housed grades 10-12.  My sophomore year, in a class with students from all three grades, we had some free time. Sean, a senior boy, asked me if I could help him with his math homework. I told him that I was only in Algebra. He said that was alright, because they were just working with matrices. I'm guessing he was taking the Concurrent Enrollment College Algebra class.

So I went to have a look. After he explained what he was trying to do and where his trouble was, I saw what he needed to do almost immediately.