Elementary School
The lifecycle begins and ends somewhere, or rather continues. One day you’re a baby the next – a grandpa. Just like people every other living organism has a life cycle. As they go through each phase of growth and life they are expected to adhere to age-appropriate norms. In this case, that is to go to school.
A sapling is an equivalent of a school-going, child – a sapling to be specific. That means they have to go to school (elementary school). But, unlike humans, saplings have to attend “Elementree” school. From there on it probably takes a couple of years or months before they attend Middle school. That is whatever version of “middle school”, trees have.