That the students actually studies and payed attention in class. That the teachers actually did their job and taught the curriculum.It's become more secretive and they can not cheat because if they do their test will not be graded, and if they do not know the answer, they will have to make an educated guess, and proving they at least know the concept.
One question that is administered to students statewide simply cannot give an accurate indication of how well a student can perform. Standardized testing also relies heavily on the fact that teachers do their jobs "correctly". A standardized test only tests few areas of learning whereas students may be much more talented in a more essential area of learning.
Its not simply because standardized testing generally only tests a small number of areas, in a very small number of subjects. There is a lot to do with academics that is absent from standardized testing,. If a person is really good at something, anything, they just have one particular skill that they are really good at, that is a part of academia, then what does it matter the scores they got on a state regulated test.