Hey howdy hey! It's time for brifraz's once every 3-4 months driveby posting... hope all is well with everyone.
As a veteran teacher who is married to another veteran teacher, I always need to chime in on these things... Sadly, most districts that are moving into teacher evaluation through student achievement are not doing it nearly as logically as what you have described. 3 year years of data which include student success before and after would definitely be a great way to approach things. And, I would have no problem with that. But most districts/counties around here are looking to tie salary to student achievement and success and you are spot on - that's a recipe for disaster. They are also offering bonuses to teachers willing to move to lower achieving schools in some of the same districts AND not looking at multiple year sample sizes. So, we are getting good teachers moving to struggling schools both altruistically and for $$ who then are in danger of getting fired the next year b/c of outside factors that might be the actual reason a school is struggling with test scores. Ack!
Lucky for us, my wife and I are both got into a good system that treats it's teachers with some degree of respect (on the negative side, we both took almost $10k paycuts to get that respect, but IT WAS WORTH IT). Regardless, it's 8 years until we retire to Celebration, FL.