You are lifting too much if you think you may become injurred. I do sets of six all the time. In fact half of the sets I do are of six. 15 is a warm up set
15 is usually a warm up set.. it also depends on your age, if you're still growing you should stay FAR away from your 1RM. but if you're over 21 then i'd say you're fine @ whatever weight you do, as long as you warm up, stretch, and have good form.
I'd say don't go too crazy. I'm also seventeen and weigh around 140. I do one set of 10 to warm up, two sets of 6, and two sets of five, and I continue to grow. I think you're okay.