I think the biggest question that you have to ask yourself in relation to this book is whether Sam is actually presented as being a character that is missing out in terms of his education by his decision to forego school and live in the great outdoors by himself. This novel very interestingly seems to compare the formal education system with the wider education that life itself can give you, and it seems to find the formal education system lacking in a number of areas. If you have any doubts about this, consider how many boys Sam's age would be able to live by themselves and hunt and cook and take care of themselves for an entire year, enduring a harsh winter, and still maintain their health and survive. Although Sam misses out on various aspects of schooling such as Maths and English, his knowledge of nature, plants and science is incredibly superior to anything that he could ever have learnt at school.
Therefore, the decision of Sam to not go to school and to experience nature, in my opinion, was an excellent one. I don't think that Sam should stay away from school forever, as receiving a formal education is very important, but Sam's experiences and development in this novel shows us the way that formal education in the school system is only part of what life has to teach us.