This is an interesting question. A coach can always schedule more games if the team hasn't reached the limit of 27. For any team that hasn't played 27 games, there is no doubt the h.s. coach can keep the kids. I have to admit, I have toyed with the notion of scheduling a game like 2 days before the State and just having 15 or so practices in late May/early June. I haven't done it because I know the girls would hate it and it would negatively affect their travel teams, upon which I so much rely to build my program. Those 15 outdoor practices would be huge, though, for teaching some of the finer details of the game that we often can't get to otherwise.
Now for teams that have played 27 regular season games, I am not sure the rule is clear. I know there is no such thing as a formal "release". little omar posted the one bylaw and that seems to take care of it, but what if a team has completed its schedule, but the coach says he/she is going to schedule another game? Has that team completed its schedule at that point?
I think the real reason a few coaches won't let their kids go is that they think the kids need a break and that they are protecting the kids from their travel teams. A nice thought, but one that should be dealt with by parents, in my opinion.