Here is a short, but interesting article on the effects of teacher observations on student achievement. In summary, the authors concluded that providing specific formative feedback has shown to improve teacher practice and therefore students achievement has improved.
Do Better Evaluations Drive Better Results?
One of the only studies to date to examine this hypothesis looked at performance trajectories of mid-career teachers in Cincinnati who participated in an evaluation system that employed multiple, structured observations done by experienced peers from other schools (Taylor & Tyler, 2012). Teachers with previously low levels of student achievement showed larger gains in performance. Researchers attributed these gains to teachers receiving, on four separate occasions, detailed formative feedback on their performance and having opportunities to reflect on—and converse with peers about—their practice.