A school is much more than just an organization where students are taught academics. It is a small world in itself. A child can not be put in front of the world right away, so they are sent to school. It is a place where they can feel safe. School definitely teaches us a lot academically but it teaches us so much more than that.
To make friends, to deal with people who have very different ideas and backgrounds than ours. It teaches us all the prerequisite knowledge we require before facing the actual world outside.
Personally, it is a great experience to remain in the same school for a very long time as it is the surroundings which form our personality and being within the same place really helps in learning while enjoying the classmates and teachers we can connect with so easily.