If you are looking for online volunteering, here are some suggestions:

Project Gutenberg is always looking for online volunteers to proofread texts which are made freely available to the world. Volunteering for Project Gutenberg is something you can do from your home.
http://www.gutenberg.org/

LibriVox is a nonprofit effort that mobilizes volunteers to record chapters of books in the public domain. LibriVox then publishes the audio files on the Internet, via its web site. The goal is to record al books in the public domain! Volunteering for LibriVox is something you can do from your home. You can listen to free audiobooks recorded by volunteers, or volunteer to record yourself reading a chapter of a book in the public domain to share with everyone, by going to the LibriVox web site:
http://librivox.org/

The Online Volunteering service (http://www.onlinevolunteering.org) has opportunities with organizations focused on the developing world. Tasks require a certain level of expertise, however (web design, translation, online research, editing, writing, etc.).

Be sure to talk with the principal to get input and ideas for you'll need all the cooperation you can get. The PTA there may be able to chip in some money for art supplies or the such, or maybe you can get some stores to donate some things.

A lot of time you can help by just getting after school sports going. Baseball, football, basketball really don't take much expertise to teach, and sometime the rough and tumble activity helps the kids lots.

You need to have it on a very regular schedule and over a long period of time for it to be effective. It will be very time consuming but I bet very rewarding.