My older two sons (22 and 19 years-old) were given minimal instruction on keeping their head up while tackling.
My son who plays in little league today goes through about 15 minutes of slow motion tackling in every single practice. It seemed like a waste of time to me at first because practices last only 2 hours, but repeatedly going through proper motion over and over again will probably help prevent what we saw with Shazier.
I think it's probably more difficult to relearn after years of doing it wrong.