Probably the best known report supporting this view is the McKinsey study of the world’s high performing schools which contains the much repeated quote “the quality of an education system cannot exceed the quality of its teachers”.
Some countries (like Finland) attain this high quality by having very high academic entry standards. The current British government is trying to do something similar by raising the bar on entry (in England) and providing a range of alternative entry routes for some highly academically qualified teacher candidates (e.g. Teach First).
However, raising teaching quality just through better ‘quality’ of new entrants would take, according to experts like Dylan Wiliam of the Institute of Education, a "generation just to close the gap on Finland by 20%". You can read the report here.
Professional learning and development of existing teachers and support for it is therefore key to improving teaching and learning in individual schools and system-wide. Hang on a minute, you might say, the McKinsey report and the one they followed up with, also highlighted the key role of effective leadership. There is a lot of evidence about what leaders contribute to pupil learning - helpfully, if rather densely, synthesised in a major international Best Evidence Synthesis “School leadership and student outcomes: Identifying what works and why”.
Happily, you can find CUREE’s much more accessible summary of the report here.The ‘killer fact’ in this study was that the activities of “promoting and participating in teacher learning and development” by leaders had an effect size of 0.84. This is twice as large as the next most significant leadership activity – “planning, coordinating and evaluating teaching and the curriculum”.