Pitching Change Situation

[rocks99]

I manage a fall team (mostly 15/16) playing under High School rules. Last evening we had this scenario.
My starting pitcher had gone 3 innings and was going out to start the 4th inning. My 1B was trotting across the field going to his position to begin warming up the infield. On the way he decided to throw a pitch to our catcher. He used the infield ball, not the game ball that was laying on the mound. The umpire immed. called me out and indicated that my 1B had to face at least one batter since he threw a warmup pitch.

Is there any rule to back up his ruling ? Any help would be appreciated.