Bill Gates put a lot of emphasis on saying successful teachers can be measured by the test scores their kids get, and their methods should be applauded and studied by other teachers. He suggested that digital video cameras be set up in classrooms so that they can see study with colleagues different successes and failures, and discuss with open honesty and with no fear of reprisal ways to improve teaching performance.
I don't think that you could mandate cameras in teacher's classrooms, as I think there is a great potential for misuse, such as fact finding missions against teachers that administrators just don't get along with.
The single greatest thing you could do to improve teaching performance is to remove everything from a teacher's workload that isn't related to teaching and allocate that to someone else. Then have the teachers just focus on those teachable moments in class. This is where teachers are most aware of who is "getting it" in class and who is not. If we are in a rush to get the next assignment out, so we can get it back and put it on the pile of marking that we already have (ensuring we have lots of data to back up what we as professionals KNOW about our students), then we often miss the students who are merely nodding along and then slip through the cracks. We assign a lot to cover our butts!
I think we should focus on marking fewer (but more multidisciplinary) assignments so you can turn work around faster in class. Kids should learn to meet deadlines, and with fewer assignments there is a greater chance they will learn the lessons intended.
That's it.
Don't ask teachers to do inventory, clean the school, repair things, or sit on too many committees. Some of this is necessary, but some of it is not. This is particularly true at the elementary school level, where there are fewer staff members, but still lots of duties to do. Often these non-teaching related duties fall on the teachers because there is not a lot of money for extra staffing.
If all teachers thought about was "what do I need to get across to these students" and then ask themselves "do my students have it now?" then I could improve school performance across our country.