Right where to begin...
In actual fact if your clinics were worth while they would be saying that the trail knee needs to have already passed the stance knee at vertical. This is a timing issue and is the result of the athletes not working correctly on front side mechanics and perhaps pushing out the rear too much
As a coach you need to observe their hip position, rotation etc, strong core will aid this.
cue the athletes to not push out the rear!!!!!!!! most important.
As for the timing issue this is where mini hurdles come in to play (can be done with drills but to get the feeling hurdles work best to begin)
Now we are not talking about running wickets here to begin with! Place them about 2 feet apart and have them do simple switches, switches are done fast, they are changing leg as fast as possible and then sticking the landing to begin with, wait a second and then go again over the next (stand on one leg, jump/switch over the hurdle (change leg whilst moving over)). what you will see in slow motion is as the stance leg is landing the opposite knee is way ahead :-) Over time this helps cue the athletes neuro system and of course then you can move the hurdles out in distance and start running wickets and voila!
(the action of stepping over the hurdle from being slightly behind will give the feeling of front side mechanics whilst the speed of bringing the trail leg through is the timing we talk about!)
You now have correct front side mechanics and timing
Hope that helps
I'm a masters sprinter and coach
www.master-athlete.com