Summer vacation has an effect size of d=.02 on learning, which is not good. .4 = 1 year of growth
But here is the deal, the longer we stop doing something, the less proficient we are at doing it. Think balancing a chemical equation in chemistry or solving the a Lorentz time dilation problem in physics, or remembering the plot of Snow White (assuming you haven't seen in 10 years).
Sure kids forget, we all do, but it is easy to dive back in and strengthen those memories with review, just like exercising a muscle.
To me the point of education should be this, teach kids to love learning, be curious, and learn how to learn. As a teacher, if you have done this, you have done your job. The goal of teaching is not to turn kids into homework machines that suck the life out of them so they can perform on the standardized test, all the while making them hate school and learning. Anything you learn today is obsolete in less than 4 years anyway and many things forced on kids in schools via state standard wish-lists are useless.
Childhood is a precious time where we learn lots and lots of stuff without sitting quietly in a desk. We build, we play, we explore the world, we ride bikes, dance, sing, play with dad's tools, and make all sorts of discoveries which aren't covered on standardized tests.
So it comes down to this, do we want study machines or children? Ask the children in South Korea.
Scroll down, school is like prison.