I don't know, nor do I think a good metric necessarily exists. Teaching doesn't fit in neatly with the typical production metrics we use in industry, eg. widgets per hour. Those outputs are directly measurable.

But what is the unit of output for a teacher? Test scores could only hope to measure teaching quality indirectly. It'd be like using record sales to assess the quality of a musician... and we'd all agree that Britney and Mariah aren't the best musicians going around.