The old way had the teacher directly teach the older kids an the age rage, who would then be responsible for teaching the younger kids themselves. This is a great system: you learn better through mentoring, you develop better critical thinking skills when the person teaching you is sometimes wrong, and you likely develop leadership skills along the way.
Having spent part of my time in a system set up like you describe... it's the *ABSOLUTELY WORST* thing you can do to a high achieving kid: take away their opportunity to reach even greater heights, in exchange for keeping them busy by becoming an unpaid teaching assistant.
Thankfully, it really didn't work out (having a 4th grader teach 6th graders math just gets that 4th grader beat up during lunch and after school), and they backed off eventually. Which was fine with me, because I was already working on calculus, organic chemistry, and college level reading that the bookmobile lady snuck me after doubling my number of books checked out quota over everyone elses.
If you want to go back to the "Little House On The Prairie"-style one room schoolhouse, good on you, but please do not drag high achieving kids back there with you, or worse try to "socialize them at their grade level", because I'm telling you, you might as well buy them a T-Shirt with a target on it.
Musk may not being anything new -- and he's really, reading the 3 articles, just describing Montessori with a couple of tweaks, like taking the grade level away -- but at least at his school I don't think you'd be holding back those who are able to vastly outpace the slower learners.