How to not get as tired while rock climbing?

So I'm on a rock climbing team and I've been climbing on and off for a few years. I'm about a 5.8-5.9 climber, 5.9 is my max. I'm 13. But lately I've been noticing that after about a solid half hour of steady climbing (not including a traverse warm up and stretching) my forearms are DEAD and I can barely climb for the other hour and a half of the class! How can I improve my stamina or not get tired as easily?