I have been running for over 50 years and I no longer lift my knees or bend my legs like I used to. I have been reduced to a slow, stiff, weak and feeble racewalker stride
The reasons why I now 'run' like this are likely the same reasons why you have high school kids running like this.
Reasons include lack of muscle strength, power, explosiveness and flexibility.
Check their standing vertical jump. Running fast is a series of small jumps put together. If they have a low vertical jump they will have a short stride.
Check their balance and general athleticism. If they are unsteady on the ground they will be reluctant to be airborne while running.
Check flexibility especially ankle flexibility.
Kids that lived a sedentary lifestyle, especially if overweight, are not suddenly going to develop the strength and power by running distance. You get kids that come out for xc that have never played any sport, and been remotely athletic, before. Distance may only result in them shuffling even more, or shuffling at the same speed more efficiently.
Work on very short sprints (< 10 seconds) both uphill and also some downhill (on soft but stable surface) emphasizing good form, explosiveness and quickness (short ground contact time).
Do drills including bounding, skipping, jumps, plyometrics.
Work on flexibility especially dynamic flexibility.
Recognize that you might not be able to improve their form or even XC time during a season or even 4 years of high school. Running form is not everything.
Ensure you know and understand the motivation behind why every runner on your team is participating, and tailor their program so they experience what they want as improvement and success.