...perfectly healthy; perfectly normal.
I think that sex one of the best preoccupations of the mind... and body!
Unless, of course, it becomes a distraction that compromises your safety and well being.
No, it is not bad. However, on your profile you have set your age at 26 years, but it does not make any difference, thinking about sex is always good and healthy, regardless of the age:)
No this is normal. You are a teenager. I used to feel that way quite a bit too, especially at school in really boring classes my mind would wander and I would feel aroused. It is incredibly frustrating especially in the middle of a boring mathematics class.