I don't mean to single out Mark. Every Democrat makes this argument.
A reduction in the number of teachers only indicates that you need more money if the reduction comes from teachers quitting their jobs. If you are laying off teachers, that shows that you are making a choice to keep their compensation too high rather than have more on staff.
Unfortunately, nobody in the media will confront the people who are saying that governments need more money for teachers. Just once, I would like to hear somebody challenge President Obama or any other Democrat on this point. This is one of the subtle ways in which media bias works, as Megan McArdle points out. If conservatives were to make an equally stupid argument, they would be called out in the media. But liberals go scot-free.