I haven't written a Symphony yet in my 5 years of writing music and there's a reason for that: they're something that I want to be absolutely perfect!
This was great, each section sounded true to the show in some way, shape or form! The opening was nice, as was the Art of the Dress section!
Here are some ideas to aid you in the future:
I didn't feel any kind of form with this...now, whilst that can be a good thing, it kind of felt like a run through of each song from the show, which does not make a symphony...this was more a big piece with 'symphony' in the title...I was expecting more of the symphonic techniques, especially because you announce the key, it gives the impression that you're following all the rules!
The flow to each section didn't, well, flow right...some parts it was an obvious change, whereas some parts it flowed very nicely! Looking at other symphonies, the form is masked very well because each section flows so nicely...and even the sudden changes feel like they should be there, not forced...that's the sign of a good symphony!
More key changes would have been nice, that would have made the piece more interesting...or better yet, non-chord tones!
More changes in instrumentation, for the most part the strings dominated throughout, which again, isn't necessarily a bad thing...but more limelight to other instruments would be good! (I must say though, the sections that did shift the limelight were nice!)
Counterpoint was basic for the majority of the time and the good counterpoint that I heard was from the actual song playing at the time...
I know it's difficult to keep up with a 20 minute piece, but there were some dodgy legato parts here and there...

I suppose immediately judging it against my two favourite symphony-esque pieces:
Per Norgard's 3rd Symphony and
Bartok's Concerto for Orchestra
was not very kind to say the least...still, it was a good piece and congratulations for making it so long!
I apologise for sounding harsh, but entering the symphonic repertoire is a big step! Gotta keep the quality high!
GOOD JOB!