Because they know that many kids aren't going to pass the test and that parents won't support schools where the majority of students fail.
This goes on all over the country, CA and TX especially. TX has been especially clever about misleading parents and taxpayers about test scores. Figures don't lie, but liars figure. Under NCLB schools can close, teachers and administrators can lose their jobs, and students can be transferred if not enough kids pass the big test.
Many states are clever about how they decide if a school is "persistently dangerous" also. I read that in NY, if a girl is raped in a stairwell, that doesn't count unless the perpetrator holds a gun to her head.
School officials, unions, and politicians learn how to play the game very quickly.