at school指的是學生在學校上課。at the school是指“在某一所具體的學校”，可以是在那所學校上班或者從事其他活動，不一定是要上學。at school也可泛指，而in the school則特指某一所學校。
at school例句：
It's really nice，as you can tell，to be at a school in such a great location。所以，學校能夠在這麼好的一個位置是很值得高興的。
What do you think is the most important thing that parents should teach kids to be prepared to listen to teachers at school？你覺得什麼是家長最應該教給孩子，讓以便他們能做好準備在學校裏聽老師教導？
at the school例句：
Have you come to register at the school？你來學校註冊了嗎？
I thought I'd enrol you with an art group at the school。我想我會吸收你參加學校的一個藝術團。
I want my son to wear the same clothes as everyone else at the school。我想讓兒子和學校裏其他人穿得一樣。
