It would help if you named what show you are doing. You may not want a “classical” song at all, but something that is from a more traditional style musical versus all the pop-style musicals around today. “Les Miserables” tends to be overdone, and it is musical theatre. That is also true of “Phantom of the Opera” not being an actual opera even though the word “opera” is in it.

I wish you had mentioned the show you plan to try out for so I could select something suitable. You only have 16 measures to show off your voice, and that is very, very little actual singing. You will need to find the right sheet music in the correct key and mark it accordingly where you want to start and stop.

For actual Renaissance-type songs—you might pick something like
“Greensleeves” or “Scarborough Fair” or even songs by John Dowland
like “Come Again, Sweet Love” or “Flow My Tears”. Dowland is definitely classical, comes in different keys, and should be okay for a young mezzo.

You might also consider songs from operettas—not really from the 1500s–but the genre splits the difference between musical theatre and opera and would be suitable for auditions for either kind of show (if only you mentioned what)!

Try “When a Merry Maiden Marries” from “The Gondiliers”by Gilbert and Sullivan,.

Here are some possible sources for sheet music
http://artsongcentral.com/song-index/ (free)
http://www.justsheetmusic.com/sheet-music/a-sullivan-when-a-merry-maiden-marries/ (links to buy sheet music)