Thank you for your help. One more question, I have.
1. I go to a middle school.
[Does 'I' mean that he is a middle school student or not?']
2. I go to middle school.
[Here, the subject "I" is a middle school student.]
3. I go to the middle school.
[Does 'I' mean that he is a middle school student or not?']
1. Yes, your interpretation is right. He is a student in a middle school somewhere.
2. and 3. Yes, your interpretation is right.