at school有以下几种意思，需要根据特定语境作出正确理解。

(1)在学校

At school, a girl may tower over most boys her age.  

在学校，一个女孩可能比她同龄的大多数男孩子都要高出许多。

He was bullied by the older boys at school.

他在学校里受到大孩子的欺负。

We've been friends ever since we met at school.

自从我们在学校认识之后，我们就一直是好朋友。

(2)在上课

The kids are now at school.

孩子们正在学校上课。

(3)在上学；求学

 Parents are positively encouraged to be at school whenever possible.  

我们非常鼓励父母无论何时，只要可能就去求学。

Bill took up art while at school.

比尔在上学期间学习美术。

Is Peggy still at school, or has she left?

佩吉是仍在上学还是已毕业了。

Attendance at school is demanded by law.
法律规定儿童必须上学。

I've kept up a friendship with a girl I was at school with twenty years ago.
我一直同工同20年前上学时的一位女同学保持着友谊。

I've kept up a friendship with a girl I was at school with twenty years ago.
我一直同工同20年前上学时的一位女同学保持着友谊。

(4)学习成绩

They are proud that she is doing well at school.  

他们为她出色的学习成绩感到骄傲。

Tom's work at school is above the average.
汤姆的功课是中上等水平。

How is Jim getting on at school?

吉姆的学业有进步吗?

I was terrible at school and left with few academic qualifications.  

我的学习成绩一塌糊涂，离校时几乎没有学历。

I got left behind at school with the maths.  

在校时我的数学成绩跟不上。

at school, in school;at the school;in the school

这两个短语意思不同:at school常指“在学校上课”,涉及的时间较短; in school常指“在学校求学”,涉及的时间相对较长,at the school指在学校里=in the school。

A:at school

The kids are now at school.

孩子们正在学校上课。(还未回来)

B:in schol

My son, though 32, is still in school.

我的儿子虽然已32岁了,但还在学校 求学。

You will find a wide variety of choices available in school cafeterias.  

在学校的自助餐厅里你会发现有很多饮食可供选择。

In school, you knew if you misbehaved you would get the cane.  

在学校不守规矩是要挨藤条的。

In school, he was not one of the achievers.  

上学时他不属于拔尖的那类。

They were more interested in courtship and cars than in school.  

比起上学，他们对谈恋爱和汽车更感兴趣。

C:at the school

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。

The queue for places at the school has never been longer.  

排队申请上这所学校的人数达到历史最高峰。

Police are investigating the suspected drug ring at the school.  

警方正在调查这所学校的可疑贩毒团伙。

Shortly after my arrival at the school, I was befriended by an older girl.  

我到学校后不久便得到了一位年龄较大的女孩友善对待。

Trouble has blown up at the school.  

学校里开始出现麻烦.

We all board in at the school.  

我们都在学校里膳宿.

D:in the school

She sings in the school choir.  

她是校合唱队的成员。

She knows every student in the school.  

她认识学校里的每一个学生。

She plays the flute in the school orchestra.  

她在校管弦乐队里吹长笛。

He teaches English in the school all the while.  

他一直在这所学校里教英语.

Girls and boys are playing in the school grounds.  

男女学生们正在校园内玩耍.