Describing a person

I can still remember my physics teacher, when I was a child and I was studying at a secondary school. It was in my first course after primary school.
This teacher looked really serious at first, and very strict. You could pass one hour, sit down in your chair, and not even don't blink an eye. Even some students even used to call him a/the crazy physicist, because he was very slim, maybe skinny; he had a deep, dark look, grey careless messy hair, and he looked very nervous all the time.
I'm not sure, but I believe he could do you become in make you a good student, because you didn't have to just know the lessons, you had to understand the lessons.
In spite of that, he recognized the worth of a student when he was a hard worker.
In my opinion, I guess he only made his job, perhaps he didn't give importance (unclear meaning), but some people need to know that his their teachers could get angry if their students they don't do their best.
I finished my secondary school and I went at the university to college… in fact, it is has been a long time from these those days, but I still think highly of this teacher.