Depending on what you mean by "school":
* If you mean the building: The opposite could be home, work, nature, or any other place you spend your time.
* If you mean the institution: The opposite could be self-education, apprenticeship, life experience, or any other way of acquiring knowledge and skills.
* If you mean the learning process: The opposite could be ignorance, apathy, laziness, or any state of not seeking or acquiring knowledge.
* If you mean the social aspect: The opposite could be solitude, independence, isolation, or any situation where you are not interacting with others in a structured learning environment.
Ultimately, the best opposite for "school" depends on the context and what you are trying to express.