*** NOT A TEACHER ***
1.) Use "at school" when you are thinking of the school as a place or when you give the name of a school:
a) John will be in school for one more year.
b) John is away at school right now, but he'll be home in May.
c) John is majoring in physics at Birmingham City College.
2.)
Do NOT use the when you are thinking of the idea of places like school, prison, church, etc. and what they are used for. :idea:
a) A child goes to school. (as a student)
b) Mrs. Jones went to the school (as a visitor, not as a pupil) to meet her son's teacher.