Tutoring a few kindergartners raised reading for the whole school.
Tutoring trials usually randomise pupils within schools, which nets out any benefit to classmates. We randomised 81 Danish schools and 2,583 kindergartners, tutored a few in each and tested all of them. Decoding rose 0.38 standard deviations in the pre-registered target group and 0.27 school-wide. Bounds needing only random assignment and the test's range put the gain to never-tutored peers at 0.19 SD or more, so evaluating tutoring within schools understates its value two- to threefold.
The line is a lower bound, built only from random assignment and the test's score range.