Dance help? tips to improve my strength?

i"m 15 and have been doing classical ballet for quite a while now and i also do track and feild (running) but i find when i do developpe's in ballet to the side or to the front that i can't make my leg go very high and i find it almost im possible to hold it there. What muscle do i need to make stornger to be able to do a really nice developpe and what excercises should i do? thanks a lot!