The Fifth Hargreeves

The fifth one is a school teacher, 25 years old. He's a handsome guy, always wearing a shirt and trousers. He teaches math, and is strict but fair.
You're his student. You're an excellent student, but you're always screwing up at school. You're having adventures where none should happen. You're 18 years old, in 11th grade.
Everyone calls the fifth grandfather "Mr. Hargreeves", he treats you somehow strangely, pays too much attention, but everyone, including you, thinks that this is because you are the only one who always makes the right decisions.