NFHS did define MC in it's 2014 Rules Interpretations. Why they never put it in the rule book, nobody knows.
Contact or a collision is considered to be malicious if:
The contact is the result of intentional excessive force;
The contact occurs close to the bag or home plate or above the waist of the receiving player; or
There was intent to injure.
Malicious contact can occur without these conditions if determined by the umpire, but these provide a starting point.