How do I strengthen my muscles while minimizing weight-gain?

I'm a 15 y/o male, and I do high school wrestling, currently in the 54 kg (~119 lbs) weight class (my actual weight fluctuates between 53.5 and 55.5). I'm trying to improve my strength, but if possible I want to stay under 57.5 kg (~127 lbs) by next season (am I still growing a little bit, I don't actually expect to gain 8 lbs of muscle by working out for a couple of months). What are some general tips for building muscle strength and/or endurance with relatively low weight-gain? General advice as well as specific exercises would be appreciated, thanks!