There are ways to assess a school, other than giving the students standardized tests.

However, the focus on assessing schools, and assessing teachers, misses the point that many schools in the US don't get enough funds. Moreover, the children in them are growing up in poverty, which means they are also under daily stress. It is very hard for a school to overcome the mind-damaging effects of such upbringing.

In effect, arguing about assessing the school is a way to draw the conversation away from those issues.