And less than you'd think.
It's hard to tamper with a lot of the scores. You don't get that much time with the answer sheets, if the school has its testing system set up according to the rules. Most of the time the teachers aren't even supposed to see the test questions.
What you *do* get is a lot of funny test prepping just before the tests. You highlight what you know from previous released tests and cram that into a 1-3 week review session. It doesn't show what they've learned. It shows what you shoved into their minds short-term and they haven't managed to forget yet. This isn't learning. This is futzing with short-term memory. It happens mostly with low-achieving schools, and in high achieving schools it happens with low-achieving students.
A key to this is that you use as many of the released test questions as possible. Since they recycle or minimally alter quite a few test questions from year to year, you basically don't teach to the test--you actually teach the test.
A test is supposed to sample the curriculum, not *be* the curriculum.