Hi Evan--
Hey, good for you that you don't drink, do drugs, or smoke!
So, have you been eating more salty snacks and fast food lately? If yes, it would be a good idea to cut down.
Like kenkeith said, often those machines aren't correct. If your doctor's office or clinic has a free blood pressure check, I would try to get there, though, and see what the reading says. Or if your school has a school nurse, you could ask if s/he could check your blood pressure--because if it is high blood pressure, you will want to bring it down.
Our 17-year-old nephew was staying with us a couple years ago and at the time, he was normal weight, even on the thin side of normal range. We took him for a physical and we were surprised to learn that he had high blood pressure; also checked it again at home on my home monitor a number of times (while he was more relaxed) and it was still high. He starting eating more fruits and vegetables, less salty stuff, laid off the salt shaker, and slowly cut down on the caffeine-containing sodas and energy drinks he was getting at work, and he started a walking program. His blood pressure came down.
Good luck!
The public blood pressure machines are not a very good source to determine the true blood pressure. It appears you are doing everything right so the probability is in your favor for normal bp.