I want to warn you; you will not enjoy my reply..........
There is nothing you can do, and to "give your daughter a very hard time about it" will only make the situation worse for your VolleyBaby. There is no "common ground" because the high school coach will do what they want, and since this is not Club Volleyball, you have no recourse. I surmise that you are making the volleyball day miserable for yourself and your daughter by trying to control a situation for which you have no control over. The high school coach is going to do what the high school coach is going to do.
The coach seems to be an old school coach - back in the day's of our youth and gas being a $1.00 a gallon, volleyball coaches rarely subbed; the six players played all rotations, with the occasional horrid ball control MB coming out back row. The Libero was not anywhere to found in the volleyball dictionary, and the defensive specialist was some short player who could not hit at all but the coach liked for some reason, so they got some random playing time.
A strength of this system is that if you have six rotations of players with ball control and attacking, it allows for a very good rhythm during games - Players don't get warm, then get cold, come in and go out. The downside is that this limits the number of players who play each game.
Don't judge the talents of your daughter or internalize this situation, because of the system which the High School coach is using. It is the system, and if the coach is winning with the system and treats their players with respect (which is different than playing time or a player/parents being happy, then it is a good system. If the coach was using a traditional 6-2 system (where the setters also hit), then there would be Opposite players which would be upset and small setters would be upset.
You cannot get mad at the coach or give your daughter grief because the coach uses a specific system; it is just volleyball c'est la vie. I have two suggestions:
1. Emphasize to your daughter the importance of practice and using those touches in practice to prepare for the few opportunities of playing time; prepare to succeed. Court time is precious, so she needs to be sharp each time she gets on the court. Also, high school practice will prepare her for club season.
2. Play Club Volleyball. Club has become the dominant vehicle for volleyball; skill development and collegiate opportunities. Be wise about how you select your club volleyball team. Ask specific questions about the systems they use, how my liberos/DS's will be on each team, how many tournaments they play a year, etc.
My answer may not have been what you wanted to hear, but maybe it can help you manage this situation for a healthier outlook.
Coach Matt Sonnichsen