When thinking about sprint training, don’t think solely about stride length or frequency, they are both biproducts of force application. Without getting too in depth about anything, low stride length is usaully caused by low hip mobility, faulty mechanics, or low force application. Increasing flexibility or strength levels would be a good place to start. As for sprint mechanics, a good place to learn some of the basics would be on the youtube channel athletes acceleration. There are also tons of good threads on this kind of stuff throughout the forum. I could give tips if you had video.
Also, wow as for your speed. I’m also 15 and if that is a accurate time your way faster than me.