英语中，有的形容词修饰人的时候，作定语或表语均可。但当其修饰事物名词时，只作定语，不作表语。例如：

a happy man （正）

The man is happy.（正）

a happy marriage （正）

定语是限制性质，说明事物给人的感觉，而非自身如何。

The marriage is happy. （误）

表语是描述性质，说明事物自身如何，所以错误。

以下是词典的解释：

《朗文词典》

[usually before noun] a happy time, relationship, event etc is a good one that makes you feel happy

 This has been the happiest day of my life.

 They had a very happy marriage.

 I have lots of happy memories of the place.

 The story has a happy ending, however.

 When’s the happy event (=the birth of your child)?

《柯林斯词典》

A happy time, place, or relationship is full of happy feelings and pleasant experiences, or has an atmosphere in which people feel happy.

Except for her illnesses, she had had a particularly happy childhood.

除了体弱多病外，她的童年特别幸福。

It had always been a happy place.

这里从前一直是个令人愉快的地方。

鉴于上述分析，对网友的句子判断如下：

My school life is busy （正）but happy（误）. 

The story has a happy ending. （正）

The ending is happy.（误）