Random variance likely accounts for the mistaken perception that small schools and small classes are better for students. Did Bill Gates waste a billion dollars on this misunderstanding of statistics?
The problem is that because small school don't have a lot of students, scores are much more variable. If for random reasons a few geniuses happen to enroll one year in a small school scores jump up and if a few extra dullards enroll the next year scores fall.
Thus, for purely random reasons we would expect small schools to be among the best performing schools in any givenyear. Of course we would also expect small schools to be among the worst performing schools in any given year! And in fact, once we look at all the data this is exactly what we see. The figure below shows changes in fourth grade math scores against school size. Note that small schools have more variable scores but there is no evidence at all that scores on average decrease with school size.
States like North Carolina which reward schools for big performance gains without correcting for size end up rewarding small schools for random reasons. Worst yet, the focus on small schools may actually be counter-productive because large schools do have important advantages such as being able to offer more advanced classes and better facilities.
Good teachers and principals are more important than small classes and schools -- and the smaller your classes and schools, the more good teachers and principals you need to find.
The linked article doesn't mention class size at all -- in jumped to that conclusion myself!