It seems that Florida is using a scale of 100-500. So, if little Billy scored 347 in 4th grade and then scored the same or higher in 5th grade, you can tell that Billy's 5th grade teacher taught Billy a full year's worth of 5th grade skills.
If Billy was the smartest kid in 4th grade and scored a 499, but only got a 378 in 5th grade you know that his fifth grade teacher didn't teach him enough.
if Billy is the class dunce, and his test the following year went up the teacher did her job, even if he started below average and is still below average.
As long as the merit pay is based on each student's score the merit pay is beneficial. You can then track how well a teacher is imparting knowledge in a tested subject. If a whole class has there test scores drop, the teacher is no good or the curriculum is horrid. If a couple scores fall, well that can be attribute to those kids. If the whole class improves, instead of staying the same, you have got a great teacher or a great curriculum and should try to replicate it in other classrooms.
For example
...it checks to see if a student has acquired a year's worth of knowledge, as determined by the FCAT achievement
leaves it unclear if a teacher is eligible for a bonus if a student is performing below grade level; for example is a teacher rewarded at all for bringing a 6th grader up 18 months in tested level in a year if they are still reading at the 5th grade level?
Or, is there adjustment for the difficulty of the task?
Teachers at schools like Snively, a high-poverty school with 89 percent of its students qualified for free or reduced-price lunches, can't possibly compete for the bonuses against teachers at magnet schools, said Viola Frazier, a third-grade Snively teacher.
IMO its quite possible that bringing a student reading at far below grade level at a Snively type school up half a grade level in a year is more of a teaching accomplishment than producing a full year's improvement in a student starting the the term at grade level at a school with more stringent entry requirements.