I don't want ANY coach teaching HE mechanics whether he/she is a MS, HS, TB or Rec coach. Are you implying that if you play MS school ball you will learn improper mechanics? I'm trying to understand your point.
BTW, DD's 1st yr 14u travel ball coach also teaches /coaches MS softball and is a pretty good coach. I have never seen him tell any of the pitchesr how to pitch. In fact he has been doing a great job of letting the catcher's call the game.
Back to the OP, take the most talented kids and have them practice together, play together and create team chemistry. Taking all 17 kids seems insane to me for MS ball.
There are some good school ball (MS and HS) coaches out there, but there are also a lot of hacks....and maybe 0.0001% of school ball coaches know what IR and BI even are. My DD played MS basketball and her coach had NEVER played basketball before, never coached before, he played soccer in HS and decided he wanted to coach, so the AD made him the girls basketball coach....it was painful to watch.