@scerruti @aiannazzone @martin @edutooters i think it is very difficult, and often inequitable the way we assess both teachers and students. I have worn both teacher and administrator hats and will make a few points, focusing on students as an example. I recently spoke to a reporter about this as it related to grades and standardized test scores such as the NAEP. And I should say, it's easy to assess or evaluate, it is difficult or impossible to do so holistically or equitably.
1) Many factors outside of school have significant impact on student learning and well being. These include meeting basic needs such as food. There have been lots of studies that have shown how social supports for marginalized communities have closed achievement gaps. All to say, it's very hard to learn when you are hungry, which also extends to high levels of stress, trauma etc...
2) Many standardized tests (often overblown and single data points that should be considered with many others) are given at irregular intervals, in the US for example, the CCSS and NGSS focused tests, that are done in 5th, 8th, and 11th grades. That make it very difficult to attribute where learning did or did not take place.
3) Standardized tests are a very narrow measure, the whole "teaching to the test" is that the goal of education?
4) Student well being in a school is the product of many many factors controlled by everyone from the principal who implements district policy, works to set culture to counselors, nurses (if any), librarians, janitors, office staff, and teachers. All of these can support or harm student learning to various degrees.

There is a pervasive over-ascribing of responsibility for often 'failing' student performance to teachers which is persistent and highly problematic.