How do I put on weight/muscle for rugby?

I am currently 17 and will be turing 18 in June. This season in rugby, the size difference from U17's to U19's is massive, so i decided to start going to the gym back in September. I go three times a week, and work all of my body over the course of the week. I don't seem to be putting on any muscle or weight, and its really frustrating me to see the rest of my team getting bigger bar me. I weigh about 12 stone, I'm about 6 ft 2, and i play Second Centre or Wing. I have from now until the end of August to put on as much muscle as i can. Please help?