A movie studio is not giving uspermission to use a script for a non profit,school play, but can we do it as a parody?

Parody requires that you tweak the copyrighted work a bit to make it your creation. Also, if you are not gaining commercially from the play, then you can argue that your use is "fair use" and you could get by that way. Copyright infringement requires that the infringer use it for commercial gains. Good luck.

Two things to consider:

First, the alleged "fair use" of another's creative work is a DEFENSE to a copyright infringement claim -- which means that once sued, you would have the burden to prove to a jury that although you used the studio's movie script without permission to create your play, that particular use was "fair" in light of the circumstances and so you should not be held liable for the alleged infringement. Being in that defensive situation makes it very difficult -- if not impossible -- to negotiate a favorable outcome for you if the studio sues for copyright infringement. The fact that you are on notice that you do not have the right to base a play on the movie script adds the wrinkle that you may be found liable for "willful infringement" -- which could dramatically increase the studio's monetary damages award and permits the judge to award the studio the attorneys' fees that it incurred to sue you.

Second, I'm not sure what it is that your play would "parody" or if it would even be a parody. If your intent is to provide witty, social commentary regarding THE MOVIE then perhaps the play could be a parody of the movie but if your play simply provides witty, social commentary on the subjects developed within the movie then that is a parody of those particular subjects and not the movie -- and so you would have no fair use parody defense if you based your play on the movie. Moreover, there are significant legal (and literary) differences between parody and satire and derivative works. A number of attorneys have ably discussed these differences when answering previous questions:

http://www.avvo.com/legal-answers/do-i-need-permission-fro-musical-parody--35816.html
http://www.avvo.com/legal-answers/can-i-print-and-sell-t-shirts-with-a-re-worked-ima-11724.html

At the end of the day, your good intentions on putting on a school play with the proceeds going to charity is only one limited factor when determining whether your play infringes the copyright to the movie script. As a practical matter, this is one case where it might have been better to ask forgiveness after the fact than permission beforehand. But no lawyer (including me) would ever tell you that beforehand.