Hi! 

Well you should definitely work on your splits and then your oversplits! Good hamstring flexibility will help you too (see here and here).

I also suggest you try stretching in a door frame or up a wall! Push your leg up how it would be in your high kick position and hold it and then try to push your leg a little higher. You can do this by putting a band around your foot and pulling it up too. It helps you to stretch your high kick position without having to hold it there yourself.

I also suggest you do some strengthening exercises. Lift your leg half way into your high kick position and pulse it there. Try to increase the number of pulses you do every day/few days/week. You can also try lifting it half way and holding it there for a minute and increasing the time you hold it for.

I hope this helps your high kicks and I hope you get in!