Hello! Im a 15 year old girl, and am not interested in bulking up

but I am in weight training in P.E. at school because nothing else appealed to me. As said, I dont want to bulk up, but I want to tone my muscles, and get stronger without really getting bigger (if its possible). Basically, should I do more weight/less reps, or less weight/more reps? And I read somewhere else on here that stretching helps muscles to not bulk up as much. Im only going to be in the class for a few more months, so its not a multiple year thing. Its not much time, but in the time that Ive been in the class, I have noticed more muscle, and I dont want it getting much bigger. Also, I need to find time to fit some cardio in my schedule...
Any help would be appreciated!