There are several things you can practice to increase your serve speed.
Think about tossing the ball more in front, so you bring your body weight more forward.
Practicing the pronation is another thing you can do to increase your serve speed.
Body rotation etc. etc.
But there's another biomechanical way to do this.
It's easy to learn, and it gives you incredible more serve speed.
Video 1
A lot of players are bending their knees too early and too deep, like in the video above.
In this way, all the energy in the legs is gone, and the serve speed will be low.
Video 2
Important are biomechanics, and using the plyometric principle.
In order to jump, first you have to bend your knees after which you stretch explosive your knees. The shorter and the more explosive you can perform this movement, the higher the jump will be. This increases the speed of your serve.
Eccentric movement lengthens the muscles fibers, and concentric movement shortens the muscles fibers.
Concentric muscle work (stretching knees) is increased when preceded by an active bending of the knees (eccentric action).
This phenomenon is known as the stretch-shortening cycle (SSC).
The goal of this plyometric principle with the serve is to develop more jumping power, what results in more speed of your serve.
Watch the video above, and you see the girl first finishing her toss and then only starting her short and quick knee action.
In this video the girl is hitting a flat serve.
She is landing now far inside the baseline, because of the contact point more in front
Video 3
In this video the same plyometric principle, now she is hitting a spin serve.
She is landing now just a little inside the baseline, because of the closer contact point.
I've recorded these 3 videos in one practice.
The most difficult thing is learning to serve in another rhythm.
Not directly bending your knees, but wait till you've finished your toss.
Because of this different rhythm, the speed shall be higher but in the beginning the control will be less. Keep that in mind. But no worries, once you've mastered the right rhythm, the control will come back
Try it and let me know if it works for you or your players!