Just have them do goblet squats. If they do those enough they'll learn to squat just fine. I reckon you're starting them off too advanced. Leave barbell squatting til they've a bit more experience.
I'd personally probably not bother with the deadlift either (sacrilege!). I mean, when you think about it, do they want to be dicking around doing mobility drills, hip stretches and soft tissue work or whatever to learn proper deadlift form or do they want to be lifting weights and having fun? Trying to fix someone's deadlift form takes time and I think the best thing to do is just get them lifting stuff.
Contradictory. You want to fix their squat, but don't give a rats behind about the deadlift. I wonder which move has more transfer to real life (BOTH).
It's not about exercise form. These are young lads. If they have issues squatting and pulling, that's a very bad thing. Why? It will show in everything they do, from running to jumping, walking, other sports, and even in some everyday activities like lifting things (duh). If they have problems now, and no-ones addressing them, they'll wound up like sadly many people do; beat up, sore and incapable of many fun sports and activities. Kinda like you (Kidding).
One exercise two-three times a week on the beginning of the workout wont spoil the fun. Even if you dump the deadlift, you should address these issues. Faulty muscular firing, dysfunction, stiffness (or shortness) and poor stability should be corrected.
There might be issues in the hip and lumbar spine area. I'd atleast recommend to do some glute activation and hip flexion exercises, and maybe soft tissue work all over the place. (PC and AC, TFL, lumbar spine).