Dylan William wrote a short, easily understandable book on which educational reforms at the current time seem to bring the most value in terms of student achievement. While student achievement alone tested on standard assessments is not the only outcome we want to improve in our schools, it is the basic one, and also it has the greatest impact on the students in their life (health, income, happiness, all are linked strongly to school achievement). It's based solely on American context of schools so some of the topics are not as relevant as others, but most of it is useful for the Czech republic, too.
The key lessons are really simple: while there is a big discrepancy between the best and the worst teachers, it is really hard to measure it individually and firing or rewarding the best teachers is problematic. The best strategy money and efficacy wise is to "love the one you're with" and invest in all teachers. That means not only paying them well (the minimal "dignified" wage), but also creating an environment in which every teacher not only can but also has to be improving their practice.
Left to their own devices, teachers generally don't improve much past their first couple years teaching. The suggested frameworks of professional learning communities (PLCs) is model used in the Czech context successfully in some projects (Pomáháme školám k úspěchu) or individual schools, but it's not a widely used practice.
The area where the most improvement can be made is the formative assessment, in terms of learning from standardized assessment as a benchmark for teacher's own progress, but better yet to use the short-cycle feedback assessment in their instruction and during teaching itself to adapt the teaching to what the students actually know.
The second recommendation is the more controversial one, especially in our context in the Czech republic, which calls for knowledge rich curriculum. Currently there is a trend of teaching transferable skills, rather than "hard facts", such as critical thinking, cooperation, problem solving, reading and math literacy etc., but all those skills are in fact best developed by incorporating rich network of interconnected facts from many different areas (aka general knowledge).
Definitely worth the read!