Blocking below the waist is not allowed anywhere on the field (even in the free blocking zone)
It was brought to my attention last night that the new INFC rulebook (3rd page) talks about the NFHS rule changes this year. 
The rule change for NFHS is actually just one step closer to the rules on blocking below the waist that the INFC already incorporates. 

INFC rules supersede NFHS rules in such situations as we took blocking below the waist out 2 years ago.   

On page 23:

Article 19: Blocking & the Free Blocking Zone: The free-blocking zone

is a rectangular area extending laterally 4 yards either side of the spot of the
snap and 3 yards behind each line of scrimmage. A player is in the free blocking zone when any part of his body is in the zone at the snap.
Blocking in the back is permitted in the free blocking zone when the following
conditions are met:
A. By offensive lineman who are on the line of scrimmage and in the
zone at the snap.
B. Against defensive players who are in the zone at the snap.
C. The contact is in the zone.
The free blocking zone disintegrates and the exception for the offensive lineman to block in the back is not to continue after the ball has left the zone.

An offensive or defensive player on the field of play including inside the free
blocking zone shall not initiate a clip or block below the waist of an opposing
player.

The only instance where we might see a player in the INFC down low is when the defensive lineman are staying low to hold their position, I believe some old timers call this "grabbing grass."  The defensive lineman are not allowed to shoot out into the offensive lineman's legs either as that should still be called blocking below the waist - even against the defense.  Trying to protect these kids knees at the younger ages.