I will note before the rest of this post is read that I had experienced some difficulty finding resources surrounding the age ranges I was focusing on, most of the points by the sources cited do focus on high school students however not a lot was found for middle school and some of it focuses towards college.
I've noticed when people have asked me why I like doing theatre or what it's done and doing for me I don't have a straight answer, "I enjoy it," or "It's fun for me to do," are my two usual answers. So I started looking at some studies about what theatre programs have done for students. I found a broad range of things that students have benefited from, one of the most notable which sadly fell out of the age range for this blog but I will mention anyway was a way to teach illiterate students a new way of reading in elementary school.

Alright enough pretty pictures for now,
Louis E. Carton mentions that theatre students have 25 distinct skills that a lot of employers out of high school and college look for, the most notable of which being oral communication and time budgeting skills. Yes I know we seem like most of the time we are just sitting on stage singing a song, doing a dance, and looking good in a dress (yes even us guys), but when it is taken into account what all we must learn in just two months or less once everything is broken down the time budgeting is pretty important when it gets to the casting. Often lead characters have roughly one half of the dialogue in a two hour show (basing off averages of what most shows do run for) this has to be read, memorized, placed on stage, and mixed with choreography and any singing by a day they have to be off of book which is usually 3-4 weeks after the show has been cast, in this case the actor may only be in a rehearsal two or three days out of the week meaning they have to have this all memorized and good to go by just 8 days of actual theatre time. I know this seems like a lot of time but when you realize an average rehearsal is only 2 hours with most of the scenes possibly focusing on other actors the actors have to take time in their normal day to memorize all lines and practice in their own environment.
For oral communication, that should be pretty self explanatory, I mean really all actors do is talk on stage. After going over both of the articles I think that these benefits, while Carton's article caters more towards finding careers after college, can hit the high school age range as well. Even though in some areas theatre can become a required literacy based class the benefits are still there with having to build a set, learn their lines, perform on a stage in front of hundreds of people in the course of a show, and work with a team for weeks on end these skills should still be able to benefits them throughout the rest of the high school career and later in life.
These are only some of the benefits offered to students within theatre programs, again I did have some issues finding the age ranges I really wanted in this part so to make it up here's another pretty picture (poster...thing)
Woodson, Stephani Etheridge. "Creating An Educational Theatre Program For The Twenty-First Century."
Arts Education Policy Review 105.4 (2004): 25-30.
Academic Search Premier. Web. 3 Apr. 2013.