Mayor Bloomberg and his Chancellor Joel Klein released “school report cards” which gave 50 schools a grade of “F” (99 got a “D”) The grades are based, almost entirely (85%) on a measure of how individual children improved (or didn’t) on two standardized given a year apart. Thus, the number of data points on which grade rests is really one. As a result of that one date point, children is “F” schools will be allowed to transfer out. In addition, “F” and “D” schools are at risk of being shut down by Mr Bloomberg & Mr. Klein To get a feel for how careless the evaluation system is check out this Daily News article about a school with very high scores which earned an "F"

As a result, of the paucity and unreliability of the data underlying the letter grades Leonie Haimson, writing at the Public School Parent Blog, of Class Size Matters points out:

But whether your child's school received an "A" or a "F", remember -- these grades are based 85% on one year's test scores alone . As such, experts say that the results are so unreliable as to be nothing more than a winning the lottery or a roll of the dice.
These grades are merely another simplistic, misleading attempt by this administration to look tough – and evade their own responsibility for the inadequacies of our schools