There is no ramming in NFHS. It is called butt blocking or face tackling. They both involve driving the front of the helmet into the opponent.
This is from the 2006 S&I guide published by the NFHS. The words frontal area and top are no loinger in the S&I caption although the same pictures are.
Butt Blocking in 1 and 2 and face tackling in 3 are both tactics which involve driving the face mask, frontal area or top of the helmet directly into an opponent in blocking or tackling respectively. both result in a foul for illegal helmet contact