One of my old colleague's son went to a Montessori school, which was based in a similar "no grade levels" philosophy like GP mentioned. They apparently smashed

One of my old colleague's son went to a Montessori school, which was based in a similar "no grade levels" philosophy like GP mentioned. They apparently smashed through the regular curriculum fairly quickly, then allowed the kids to pursue something that interested them. His son in particular learned how to produce music, and he was actually pretty bloody good at it for a kid. At no point did they actually need 1:1 tutoring to ram education down the kids' throat; given a little bit of free reign the kids would gladly learn things on their own.