The only reference I could find regarding length of cleats in the NFHS Rule book is:
Nonremovable cleats are limited to studs or projections that do not exceed ½ inch in length measured from the sole of the shoe to the tip of the cleat and which are made with nonabrasive rubber or rubber-type synthetic material that does not have or develop a cutting edge.
And this mention of illegal equipment:
Examples of this type of foul include, but are not limited to, the wearing of cleats that exceed ½ inch, or the presence of a sticky substance on a player’s uniform. If a player is detected wearing illegal equipment, his head coach is charged with an unsportsmanlike conduct foul under Rule 9-9.