The upshot for this is that the benefit is as much for the student doing the teaching as the one doing the learning. Teaching has a much greater effect on _retention_ than listening reading or even doing, which is the majority determinant underlying the primary school curriculum.
There are a whole host of secondary benefits to this (as well as lots of logistical challenges): the students are doing something useful, teaching, and we pay teachers if you wanted to expend budget there I suspect it would have great effect, as would any other form of ~~bribery~~, I mean, incentivisation; socialising, especially if you have the teaching being done across different classes (which you would want to do because you want the teacher to know more than the student).
In the USA there are approximately 50 million students aged 5-18. If you paid for each student to get 1:1 attention one day a week, you would need one teacher per five students in schools that meet five days a week. Let's use that number because it reduces 50 million students nicely to 10 million teachers. Let's pay each teacher $70K/year. That would cost $700 billion per year.
The USA military spent $100 billion per year in Afghanistan.
If the USA provided the 1:1 attention only in 1st Grade and 3rd Grade, they could fund it with the same commitment they made in Afghanistan with a lot fewer deaths. The USA persisted in Afghanistan for 20 years. Shall we experiment with education for 10 years and see if we get a better result than we did in Afghanistan?