This tendency to seize on a success in a limited span of the academic spectrum with a limited section of the total student population and to then apply the principles discovered to every subject and all students is typical of people from fields outside education when they venture (as they so frequently do) to publish their opinions on how to solve the problems inside American education (I haven't noted this tendency elsewhere). These opinions are almost always, in a word, simpleminded. The Economist is actually quite a sophisticated publication; but to pretend that Khan Academy-style dashboards can properly measure the success of all teachers in all subjects is simple-minded in the extreme. Exhaustively running through all the objections that might legitimately be raised to such a proceeding would pointlessly tire me out; but I will mention just a few.
The central question here is, How should a teacher's professional performance be appraised? I will quote from One World Secondary School's contract for teachers to provide a model for what I support:
The criteria for assessing this are knowledge and understanding of curricula; planning, teaching, and class management; assessment, recording, reporting, and accountability for student achievement; pastoral and co-curricular support for and involvement with students; and other professional responsibilities, including approach towards professional development as well as interaction and cooperation skills.
Of these five categories of criteria, only one has to do with student performance on assessable work, whether formative or summative, and that is the second ("assessment, recording, reporting, and accountability for student achievement"), and only a portion of this is attributable to what the students do; the vast majority of these assessment criteria are determined by what the teacher does, that is, by the one being assessed (and promoted or fired, in The Economist's vision), not by someone else. Deepening one's understanding of the curricula one teaches, planning for and managing classes, and supporting students outside of class time are also very important professional responsibilities for a school like One World Secondary, and for nearly any other secondary school as well; and yet note the qualification, for I do not believe, in spite of my support for this particular set of appraisal criteria, that it is necessarily the right answer for every school in existence; and therefore our approach to teacher appraisal, and to commenting on educational issues in general, is not so simpleminded.