Surely your teacher has offered you alternatives? Scores aren't free. Do you have options available at school?
10 minute excerpts from Candide, The Barber of Seville, Madame Butterfly, The Mikado, Oedipus Rex, Carmen, Faust ...
Free Operatic Scores (heed the languages!):
http://imslp.org/index.php?title=Special:Search&limit=20&offset=20&ns0=1&redirs=0&searchx=1&search=opera
Free Musical Scores (You will have to search titles on your own - this is for Pirates of Penzance):
http://memory.loc.gov/ammem/mussmhtml/mussmTitles367.html
American Musical Theatre collection (Library of Congress):
http://www.loc.gov/performingarts/encyclopedia/browse/ammusical.html
If you just need musical theatre pieces there are so many to choose from: West Side Story, Bye Bye Birdie, Fiddler on the Roof, Les Misérables, Cats, The Fantasticks, Showboat, Grease, The Phantom of the Opera, Rent, The King & I, Chicago, A Chorus Line, Hair, Evita, Brigadoon, Jesus Christ Superstar, A Little Night Music, Timbuktu!, The Producers... do a few ABBA songs and call it Mama Mia.. lol
Ultimately, what you choose should depend on the vocal ranges and ability of the participants.
Here's a site for you:
http://www.musicaltheatreaudition.com/shows/1showlists/index.html