Anyone who has worked with athletes knows that it is not uncommon to see the inability to simply bodyweight squat to parallel with proper form. We could argue over what the issue is. Lack of ankle mobility? Lack of hip mobility? Lack of core stability? Could be one, all, or a combination of the three.
The first, best and easiest fix? Raise the heels. Raising the heels gives you more ankle mobility. Raising the heels causes an anterior weight shift which makes it easier to sit back when you squat.
PS: Powerlifters and Olympic weightlifters have been wearing shoes with an elevated heel for a long, long time. There is no actual research that shows that elevating the heels hurts the knees.
PPS: If someone can’t perform a bodyweight squat properly, loading them with a barbell isn’t all that smart and you could seriously hurt someone. It’s actually pretty irresponsible.