He didn't specifically mention measuring test scores, you did. But is your argument that effectiveness of teachers, effective educational outcomes are unmeasurable? Then how do we know that schools aren't doing perfectly fine right now? Why is there even a discussion about improving schools?
Would it be that objective comparisons between US and foreign students is trending downward (while costs go up)? Would it be that more and more a high school diploma (then college degree) means nothing as far as employability?
Is it that the demands of modern economy is not matched up with a supply of highly skilled workers? I think these are all evidence that public education needs modified... not necessarily that someone needs "blamed" for it, just that everyone needs to acknowledge it and be supportive of change.
Now I'd argue that measuring student test scores should be one part of many that go into evaluating teachers. But above all there has to be something that we use to evaluate them, otherwise how do we know it's working... or not... or when or how to try new ideas? It's not just the teachers but the whole system that should be constantly evaluated. Not necessarily to fire someone, but to give feedback and constantly improve.
For sure it could be argued that it's not teachers themselves that are failing students, but the system is structured to be resistant to change and innovation. Metrics that should be used to guide curriculum are graduate employability, job placement, are skills being taught to satisfy the needs of the job market, what's working and what isn't etc.
Introducing some sort of competition will allow schools to figure out the evaluation process themselves. Look at places like Dev Bootcamp that claim 80% of graduates are hired with X salary after graduation. That's succinct way to measure the effectiveness of their teachers. Not that I'm saying this example exactly compares to public school or that a CS solution is applicable to the overall problem but... on a side note why isn't programming and such more prevalent in high school? There's no reason a private company can impart those skills in 9 weeks and public schools couldn't do it in a year. Maybe this type of thing has been incorporated into public schools if so disregard...it's been a while since I've been there. Generally, I think high school was an extreme waste of time in regards to what was actually learned. And then college is four more years with knowledge that itself could have been taught in high school. I tell people all the time, nothing I learned in CS major in College couldn't have been learned in high school. It just wasn't offered.
Likewise for subjects like economics, finance, personal finance, etc. Handling ones finances is something that more Americans need to learn and these lessons are not being taught early enough.