at school指的是学生在学校上课。at the school是指“在某一所具体的学校”，可以是在那所学校上班或者从事其他活动，不一定是要上学。at school也可泛指，而in the school则特指某一所学校。

at school例句：

It's really nice，as you can tell，to be at a school in such a great location。所以，学校能够在这么好的一个位置是很值得高兴的。

What do you think is the most important thing that parents should teach kids to be prepared to listen to teachers at school？你觉得什么是家长最应该教给孩子，让以便他们能做好准备在学校里听老师教导？

at the school例句：

Have you come to register at the school？你来学校注册了吗？

I thought I'd enrol you with an art group at the school。我想我会吸收你参加学校的一个艺术团。

I want my son to wear the same clothes as everyone else at the school。我想让儿子和学校里其他人穿得一样。