Whose line is it anyway?

[WhistlesAndStripes]

NFHS 3 man mechanics

After a timeout, A1 is throwing in to A2 from the end line in the backcourt. B1 is defending A2. After a successful throw in, A2 runs up into the front court where the other 7 players are. The trail official is table side watching as A2 dribbles the ball up the floor. As A2 approaches the free throw line in the backcourt, B2 knocks the ball away and it goes out of bounds across the sideline on the side of the court opposite the table.

Which official is responsible for blowing their whistle on this violation??