The best advice I can give to anyone who has written a play is
go out and do your play.
Don't sit around waiting for an agent or someone to "discover" you.
While you're doing that, someone else is already out there doing their play
and your agent is going to discover them instead.
You don't need the approval of a theatre professional to find out if your script has merit.
Show it to your best friend and ask 'em what they think.
Invite some people over to your house and read it out loud in your living room.
Or get together with some friends and just start rehearsing the play after school and
perform it at a local park, or an open mike night or in your own backyard.
Friends are important. They'll take a chance on an untested script from an unknown playwright every time.
The artistic director of the Broadway Muckety-Muck Theatre in New York New York never will.
And you will learn far, far more about what works and what doesn't work in your script
from seeing it up on its feet in front of a live audience
(whether that's an informal gathering in your own home,
or a handful of revelers at a local fringe festival)
than you will ever learn from a politely-worded rejection letter, no matter who wrote it.
The most common mistake made by young playwrights
(and old ones, too, for that matter)
is placing too much emphasis on the importance of being accepted
by "established" theatres and theatre artists.
That is something that comes at the end of your career, not the beginning.
Read up on how Mamet and Malkovitch started out performing plays in church basements in Chicago.
This is how you establish yourself as a theatre artist.
From my own experience, the beginnings of
No Shame Theatre were equally humble.
We started out performing No Shame in the back of a pickup truck
in the parking lot of the University Theatre Building
because the University wouldn't give us permission to perform inside the building.
The show developed a following nonetheless
and by the time No Shame was "officially" sanctioned by the University
it had already been written up in a national theatre journal
and launched a second branch which was performing at
the Public Theatre in New York.
Don't ask. Do.