mixing students of different academic levels boosts achievement,

This is a mixed bag, getting a lower performing student and putting him/her inside a school with high standards will increase the standards of that student. Having to many lower performing students will decrease the level of the entire class. In a way it is an experiment for the consequences of immigration.

students have been found to perform better when teachers don’t grade their work

This may be marginally true if you test on students that have already been educated using graded tests they may have slightly better results do to reduced pressure. Once you introduce new information a lot of students will not study for the test and in a longer period of time they will 100% not study for anything while blaming the teachers for their lack of basic skills.

It is also worth to mention that teaching students to manage their anxiety under pressure and focusing on the task in front of them too get optimal results is often a more important skill too learn then what they are learning in class. I respect those that manage to cheat during tests for this reason.