1. I go to a middle school.
2. I go to middle school.
3. I go to the middle school.
[What are the differences among them? Which one is commonly used?]
1. You're not referring to a middle school anyone in your audience knows about.
2. You're referring to the grade levels in middle school, but not with any specifics. Basically, the sentence is implying "I" is not in elementary or high school.
3. You're saying that "I" is going to the only middle school in town!