Question

how school is both an association and institution? discuss

Answer 1

School is designed as an association where people come with basic same interest and purpose. They perform same activities thus working as an association. 
And in this association, the knowledge that is provided to them to make them able to do better in life and for the sake of learning make it an institution for learning too. 

Answer 2

Let me try to help on this.

School is a place where learning occurs and it entails three main individuals, they include parents, teachers and students.

The relationship of the parents to the teachers is a form of an association kind  of relationship, since its through the students that parents come in.

The school is an institution because its a recognize public resource meant for learning activities. Which is recognized by both the public domain and the government.