I agree with possum. To further expand, there are many "how to" videos on YouTube. While I wouldn't recommend them to somebody attempting to learn on his/her own, you've obviously had some instruction and can use them as reference material to perform it in the manner which most closely resembles what you do in your school.

Some of your problem might stem from not building enough momentum with your head and upper body as going into the kick. Move your head and upper torso more violently, swinging it down, over and then up as you go into the kick and see if that helps. I used to play around sometimes and one day Ho San Pak saw me clowning around with this kick and gave me a few pointers. :)

To troubleshoot, we need to see what you're doing. So, post a video of what you're doing.

To have a frame of reference, we need to know what you mean by "butterfly kick", since there are styles with a "butterfly kick" but who execute it very, very differently. So, post a video of what closely approximates what you're trying to do.

Okay I'm 14, and I learned to tho a butterfly kick, bout I barely go in the air and my feet/legs don't go that high and aren't really stretched out like others when they do their butterfly kicks ...

Can anyone give me any tips to improve on this ? Please ._.

Thanks