The NFHS' version of the NFL's Illegal Contact rule is in the Defensive Illegal Use of Hands foul. As stated earlier, once it is apparent that a potential receiver is, in fact, attempting to run a passing route then Team B can not impede/hinder him in any way beyond the Line of Scrimmage. As a former BJ, I would key on if the receiver's stride was altered by the defender. If it is not obvious that a potential receiver is attempting to run a passing route then a Team B player at, or beyond the LOS, may legally block the Team A player in question but they only get one-yard of continuous contact. There is no 5-yard belt like the NFL as this contact can be any where on the field as long as the proper conditions are met.
Behind the Line of Scrimmage a potential receiver may be legally blocked at any time for any length of yardage or time, even if the ball is in the air coming towards him, because you can't have PI behind the Line of Scrimmage and the Defense may legally engage any Team A player behind the Line of Scrimmage.
The big headache in this is that often a floating in zone Line Backer will often decleat a Team A receiver in a crossing pattern before the ball is thrown. Not only would this be Defensive Illegal Use of Hands, you also have a Personal Foul for Unnecessary Roughness for an Illegal Blindside Hit, you might also have a Targeting foul to consider. Always keep an eye out on crossing patterns for this big illegal hit as well as a potential OPI for blocking on a pass downfield.