This may explain why some unions are equally well known for their lack of productivity; the American teachers' unions are generally believed (by everyone outside of the teachers' unions) to be the primary obstacle to improving America's appalling public schools.
One possibility is that, to the extent that unions do increase productivity, they do so by forcing less competent workers out of the labour market, because they are not worth union pay. In teaching, where the average wages are nothing special for the target, college educated applicant pool, this doesn't work. Indeed, by compressing wages, it makes the problem worse. In areas where there is an oversupply of graduates, such as English and history, teaching programmes choose from the applicants who have relatively few other opportunities; while in areas like science and math, where almost any qualified applicant has higher-paying alternative opportunities, they face permanent shortages.
Will market pay ever come to public school teachers? Not in my lifetime--but for a math teacher, it's a nice fantasy!
Note: this post about unions was not entirely negative, and the linked article even less so. What might that indicate about me, my leftie readers? I'll give you a hint: it rhymes with shmopen-line-ded. =)