The very last PowerBooks were sold October 2005 - January 2006. Not only that, but Apple announced the transition on 6 June 2005, or almost exactly four years ago.
If your PowerBook just expired its three year AppleCare, you must have bought your PowerBook *just* before the release of the MacBook Pros, and most certainly after Apple announced the transition.
Four years is a very long time for notebook computers. Apple gave you four years of full support after announcing they were planning on EOL PowerPC machines.
As far as security patches go, Apple continues to release updates and security patches for the previous generation OS. For example, with the release of Snow Leopard, support will cease for Tiger (OS X 10.4). Apple just last month released a security patch for Tiger (PPC). In addition, I believe all of the updates released recently for the various iLife products run on Tiger/PPC.
This means you should expect continued patches for your Leopard/PPC machine for at least the lifespan of Snow Leopard.
What confuses me most about your comment is your mention of a classroom of machines running MacOS 8. This proves that Apple software (and hardware) continues to be useful long after it has been EOL'd, but somehow your PowerBook will cease to be useful the minute Snow Leopard is released and is unavailable on your PowerBook?