I am 16 years old, and although my lower body is strong in muscle, the upper half is very weak in muscle.
Rather than lifting weights, I was wondering if punching a punch bag will actually build muscle on my shoulders and arms? If it doesn't, I'll stick to weights, but I'd much prefer to use the punchbag as I enjoy it. Thanks.

I use the punching bag as a cardio workout on my non-lifting days. I think it is a good cardio workout, but I'm not sure it will build muscle the same way a weight training program will.

what is a good thing to do as a punching bag excersise..just hit it?

Yeah, its realy hard work if you ever try

Sure, just hit it. But you can try different combinations and stuff while you're doing it. And, if you dance around like Apollo Creed, you'll be doing a lot more for your cardio workout.

I swear by my punching bag workout. I need to do it more because tones your muscles great.

Lift weights.

Continue to do what you enjoy.

using a punching bag will help you with muscular endurance, and is great for cardio, but it will not "tone" up your body. remember, your body can only do a couple things - gain/lose muscle and gain/lose fat. so, yes it would help to make your muscles show better by lowering bodyfat, but will not help you to gain muscle on your shoulders and arms.
so.....stick with weights for your main workout, but keep the punching bag in there as a form of cardio.