Can someone tell me if assistant coaches are allowed to stand in the players circle during a 30-sec timeout? Please add reference in NFHS rules book or case book.

There is no problem with bench personnel standing during a time out. They are allowed to stand in the player circle along with the head coach. 10-4-4-c which deals with technicals, states that this is one of the exceptions as to when bench personnel may stand.

An editorial change that was made this year was to take the assistant coach out of the "Coach's Rule" section of Rule 10. They aren't mentioned at all anymore, which effectively moves them into the "Bench Personnel" section, 10-4 as Ed cited. So if the trainer can't do it, if the scrubs can't do it, if the water carrier can't do it, then the assistant can't either. And of course, the converse is true. bench players are allowed to be in the huddle, so assistant coaches are, too.