And one other thing. You would be surprised how much a big powerlifting style squat DOESNT apply to a big high bar squat. I coached one kid who had just done over 600lbs in competition a month or two before starting OL, and he struggled, and I mean really struggled, to do 385lbs on a high bar squat for a couple of reps. thats not uncommon. Some of these kids have never used their quads in their life. You ask them to keep the chest up and squat all the way down, they are incredibly week. I was pretty much the same way when I started OL, had an PL backsquaqt of over 800lbs, couldnt hibar squat or front squat my way out of a wet paper bag.
But its a mistake to just hammer the back squat in this situation... they inevitably bend over every time it gets heavy... simply because that is the position they are strong in. Better to keep the back squats light or non-existent for a while, till they have built some quad strength with OHS and/or front squat, both of which pretty much FORCE you to do them right or you dump the bar.