[mbyron]

For HS varsity (or JV), I would not let the coach make comments while he's changing pitchers. For one thing, arguing balls and strikes is prohibited by rule throughout the contest. For another, it sets a bad example in front of the two players most likely to whinge about it, namely F1 and F2.

If the coach starts to make any kind of comment about the calls I'm making, I interrupt and ask: "coach, are you arguing balls and strikes?" I think that sends the message without being too confrontational. I will add, however, that in the HS games I worked in 2006, I did not have to use this strategy at all.