[Freddy]

10-5-1: "The head coach shall remain seated on the team bench, except:
a. By state association adoption, the head coach may stand within the
designated coaching box".
Rule book does not use the term "assistant coach. By inference it is clear that the coaches box, along with the privilege to stand, is afforded only to the head coach.
Had this last week. Cf. "Jack-in-the-Box" Coaching thread previously posted.
If the phraseology you're looking for shows up elsewhere, it will no doubt be posted soon by a member who knows.