I have done many dozens of classes with kids from Kindergarden to University. I got dozens+ of IBM Engineers out to do "Computer Demolition" classes in the 90's . ftp://medialab.freaknet.org/pub/documenti/compdemo.pdf
(My original old site...)
There is NO problem connecting with schools and doing short classes or demonstrations. Contact the Science / Technology / "Gifted-Talented" teachers directly. Don't ask the School Board or the Principals to start.
If you have a well-developed workshop/demo with hands-on for the kids you have it made. But talking heads don't make it.
AND: Secret#1 NO CHAIRS! (For Middle-High School kids) Seriously, try to use a space with no chairs and get the kids to move around and do stuff. I have often stacked up all the chairs in a room and used just tables.. Middle School kids can be glued to their chairs and nobody wants to make the first move. Crazy but true.
Next approach is to offer to 'help' periodically in an established class, by preparing and presenting a small section of the class (often which the teacher does not have much background in).. This is directly under the supervision of the teacher and only the most rabid Union-Dogs will growl at it.
But make contact with teachers, and also research online the kinds of activities that are already happening..
BTW, private schools and "International Schools" around the world usually have no LEGAL requirement that teachers be licensed, and are often open to well-educated people with other degrees. I've been overseas the last 7 years where my wife worked at schools in Africa, China, MiddleEast, and have often been asked to teach (status: Retired Engineer).
Also consider working on good online resources for kids Worldwide. Example: http://arduino-info.wikispaces.com/BrickStarterSet
It's really cool when you hear from some kid in Borneo or Iceland!
Let's hear from some other experiences...