On my first day at the school, I was terrified.
On my first day at school, I was terrified.
He won’t come back from the school until five.
He won’t come back from school until five.
They finish the school at that time.
They finish school at that time.
My son didn’t go to the school yesterday because he wasn’t feeling well.
My son didn’t go to school yesterday because he wasn’t feeling well.
Use school without an article when talking about its normal purpose—students going there to study: ‘at school’, ‘go to school‘, ‘come home from school’ and ‘finish school’.
Use a/the/my + school when talking about a particular school or its building: ‘The school near my house is very old.’ ‘She went to the school to speak to her son’s teacher.’
The same general rule applies to words such as university, college, bed, church and court. In British English, it also applies to hospital.
See BED (2)