I was with my Dad who is the scorekeeper for the High School hockey
games. Only the referees tell him who scored and assisted. Usually there
are two assists in every goal and rarely one or unassisted. I have seen
the assist have been award to the Goalie! (Just how many points does
each goalie have in the season?) That happen when he made save and give
it to the defense then pass to forward BOOM, scored and the referee
award goal to froward and assists to defense and goalie. My Dad said
it's up to the Referee to make the calls and that's official. After the
game, the Referee signed the score sheet and that's that.
 
I'm sure that there's a rule in the rulebook that only referees make the
official call and pass it to the scorekeepers. Not the scorekeepers make
the calls on who's the goal or assist award to.