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By "old-fashioned" comedy are you thinking a classic? Also, how big a cast?  If you have a few more details I can give some better recommendations, but for now how about...

Kaufman and Hart's "You Can't Take It With You" and "The Man Who Came to Dinner" 

"The History of Tom Jones"

"Arsenic and Old Lace" (two female leads)

Ken Ludwig's "Lend Me a Tenor," "Moon Over Buffalo," "Leading Ladies"

What about a Shakespeare? "Midsummer" or "Taming of the Shrew" or "Much Ado"

"The Complete Works of William Shakespeare (abridged)"

"The Complete History of America (Abridged)"

"The Brothers Grimm Spectaculathon"

Two comedies we had great success with were Meet Me In St. Louis and Pride and Prejudice. St. Louis provides some great physical comedy and classic comedy lines, while the humor in Prejudice doesn't really come out until performance - I remember my kids were so surprised at all the laughter during the first performance because the high British humor doesn't come out as much in rehearsal. I think both of these were versions from Dramatic Publishing.

"Fools" by Neil Simon. The lead is a male but the show is a lot of fun and my students loved it. 

I would highly recommend Stardust by Walter Kerr.  It's really funny, written and takes place in the 1940's, has a lot of characters that the students can relate to (that are also fun and complex for them to play), and has a decent sized cast: 11 female and 7 males.  We're working on it right now, and the students are having a great time.  It's available from Dramatic Publishing.

Happily Even Once Upon by Virginia Kidd. Several male parts but the majority of the show is carried by the females (although Prince Charming has a good size part)