I'm looking for a non-musical large cast comedy.  This is a high school production, and past productions have included anywhere from 30-40 performers.  I'm at a loss and looking for suggestions.  

Sixteen scripts listed here which may include something suitable. 
(Some of them may be too short, a few will be too young - "High School" can cover a broad range of maturity!  You can read them all on-line to decide.)

That's a heck of a lot of people for any play!  You can't help but end up with a lot of "spear carriers."

I would recommend "A Midsummer Night's Dream."  It's the one show I can think of that has at least a dozen really good roles for talented people plus opportunities for the "spear carrier" set with the troupe of fairies, and the people of the court.  You've got Hermia and Lysander - star crossed lovers, Demetrius and Helena - also star crossed.  Titania and Oberon - royal fairy folk on the outs with each other.  Puck - the best mischief-making elf ever written.  Then you have the "rustics" - the locals who are rehearsing a play to perform before the court, with Bottom (a great comic role), and the others all memorable: Quince, Flute, Starveling, Snug and Snout.  (Their comic version of Pyramus and Thisbe - a Romeo and Juliet type story - is one of the funniest sequences in any Shakespeare play.

If the Bard intimidates you, I've got a "simplified" version that I adapted from the original.  (The language is still there - just made a little more actor and audience friendly.)  Otherwise, check out the original, or some of the other adaptations that are available.  The best thing about the show - aside from the fact that it's genuinely funny - is the fact that there are so many good roles in it.

"The Taming of the Shrew" is another solid possibility.  It's more than just Katherine and Petruchio.  There's Bianca and Lucentio, Tranio and Biondello, Grumio, Gremio, Hortensio, Signor (or Signora) Baptista, the Pedant, and Vincentio.  Again, a wealth of "good" roles.

Or maybe "Twelfth Night" - a great gender-bending comedy with an easy eight to ten memorable roles, and lot's of courtiers, etc.

Long story short - check out Shakespeare, in one form or another.   

Delia Dancer, Doughnut Girl, from Heuer Publishing, with a flexible cast of 21.
Synopsis: Delia Dancer, Doughnut Girl is a modern melodrama that chronicles a ridiculous day and the challenges and struggles of a young woman who makes a living manufacturing doughnuts and sweet pastries in a small town doughnut factory. Structured as a radio program complete with phony commercials, parody songs, audience participation, and listener call-in spots, Delia Dancer, Doughnut Girl stretches theatrical forms but is deeply rooted in awful puns and wrapped around an apple pie and a cup of coffee of small town life.
Types: Full Length Play, Community Theatre
Genres: Comedy, Melodrama
Cast: 6 men, 7 women, 8 extras, flexible cast, 21 total

We have a number of shows that can be performed with or without music that require a large cast.
The Complete History of the World (abridged) has 50 speaking parts but some of the smaller parts can be doubled up.
Persephone and the Three Golden Hairs has 33 speaking parts - it's our newest show and is very funny indeed.
Panto Crime may also suit your cast too.