Hey, I'm scrawny(ish), but I'm not weak.
Strength and muscle don't necesarily relate. You can be strong and have no visible muscle at all.
I say you should just wait a while longer to develope your muscle mass. You're just 15, you haven't completely developed yet.
You may not be getting any taller, but your body is still growing.
Just keep working out. Getting muscle isn't easy... unless you take steroids, but by doing so, you'll prove that you truly are weak.