I can't seem to find who has the rights to Sideways
Stories From Wayside School. The children's book
was adapted by John Olive for the stage.   I've seen
photo's of theatre's productions of it on several
websites and it looks like a great show using adults
and kids.   Please help!!! Thanks.

Two suggestions:

Try the Hansen Drama Shop (www.dramashop.com) - they helped me find a title that I searched for a long time.  They went above and beyond to help me find it.  They were very prompt in their reply.  Their home page has a place to write them for help finding a script.

Try the AACT play source web site (www.aact.org) and just try each source - some specialize in plays from books and others specialize in productions for younger actors.

Good luck!!!