Here is the UIL policy:
Guidelines for Bands Playing during Football Games.
UIL Football games are conducted under NCAA Rules. As it relates to when a band may play during the game, the NCAA Football Rule Book states:
Persons subject to the rules, including bands, shall not create any noise that prohibits a team from hearing its signals (NCAA Rule 1-1-6 and UIL Rule 9-2-1-b-5).
PENALTY—Dead-ball foul. 15 yards from the succeeding spot [S7 and S27]. Automatic first down for fouls by Team B if not in conflict with other rules. Flagrant offenders, if players or substitutes, shall be disqualified [S47].
To assist with compliance, consider the following procedures:
• Have the band stop playing from the time the Offense (of either team) breaks the huddle until the ball is snapped.
• Or, in the case of Offenses that do not huddle, have the band stop playing from the time the Center on Offense is over the ball until the ball is snapped.
This may necessitate creating shorter versions of some music, as is common practice with college bands. A drum line playing throughout the game generally does not prohibit a team from hearing its signals – use your discretion.
Although most officials do warn a band prior to calling a penalty, they are not required to do so.
Consider discussing this matter with your head football coach, athletic director, and/or school administration to determine a plan that is appropriate for your school.
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I have had many conversations with band directors. It only takes one conversation, and the problem goes away.
By the way (for millennials, that means ‘BTW’), this policy is issued by the UIL to the band directors, and, as you can see, is written as though speaking directly to them. So, this is the ‘policy’ that you reference when politely notifying the band director to stop playing when the opponent is preparing to put the ball in play. Since it references the NCAA rule, and offers these procedures as something to ‘consider,’ it is still our judgment as to whether the band playing prohibits a team from hearing its signals. But, if there is any question in your mind, or the opposing teams complains, then have that conversation with the band director. Try to nip this in the bud early in the game. If they are playing while the opponent is preparing to snap the ball, at the next ‘break in the action,’ go over to the band director and remind him that, by UIL policy, they are not permitted to play while the opponent is preparing to put the ball in play. That has always worked for me.
If the opponent is complaining, stop the game and have that conversation immediately.
Don’t ‘accuse’ the other band, by association. Treat each band independently. If the other band is behaving itself, don’t ‘warn,’ them, too. In some cases, yeah, you’ll eventually have to talk to both band directors. But, no need for that unless both are misbehaving.