I recently came on as the new director at a very active-parent school. After auditions and casting for the spring musical, there was quite a bit of commotion in the community.  So, I decided to use the upcoming parent booster meeting to let the parents in on my process:
Since I didn?t know who this year?s acting/singing pool might include, I had held a ?talent-pool audition? prior to selecting the show.  Good thing, too, because no tenors (most male leads) showed up: better find a show with no tenors!

We considered ?Annie,? but they?d done that just a couple of years prior, so we selected the Broadway version of ?Cinderella,? in which Prince Topher is a baritone, as is the other male love interest, and there is no king.

Part of the decision was the input from the music director and conductor, to be sure their students had the talent we?d need for the orchestra for those shows.  Most parents think it?s a unilateral decision what show to select?not always true.

Most of the auditioners were girls. This also made ?Cinderella? a good choice. as it has really good roles for girls.  Most shows are ?guy-heavy,? something parents don?t think about one way or the other.  It?s also a huge costume show (including two miracle-fast on-stage costume changes), but we had the tech talent to pull that off.  If we hadn?t, we might have kept looking (I?m committed to a student-centered company).
Since the seniors were still wounded by the departure of my predecessor, they basically boycotted the production.  I got angry emails after the cast list went up, about denying roles to those for whom this would be their last chance!?Nobody realized those seniors hadn?t shown up to try for any roles.  Oh.
?What about MY talented daughter??? Yes, well, your daughter just wasn?t our Cinderella, and put on her audition card that she would not accept an ensemble role. Did she forget to mention that part to you??

?What?s with casting a freshman as Cinderella??!!?  I explained that instead of seniority leading to bigger and bigger roles (the previous system), I used ?character-match? to cast, and that talented young lady had the chops, the looks and the personality to play the lead more so than anyone else I saw.  Decision made.

When an entire population is cast from a four year age group, it is likely that the upperclassmen will look, for the most part, older than the younger kids, and so tend to fit the adult roles?and the younger roles tend, in general, to go to the younger actors. When casting a (wicked step-)mother and her daughters, the odds are stronger that an upperclassman will match up with underclassmen for a believable family look.
Why didn?t I cast ALL the talented, capable kids?  Two reasons: Only so many kids will fit on the stage at one time ? especially in the big poufy gowns they?ll wear at the ball, and we just can?t choreograph a mob swaying as our big ballroom production number.
Also, with so few boys auditioning, after about a 3:1 ratio of girls to guys, it begins to be noticeable, as if some plague wiped out most of the male population of the town.

The parents thanked me profusely afterwards, for explaining a thoughtful process about which they had been unaware.  Now that it made sense, they were fully behind me.  Whew!