The students are about to be shown how hard the test is.

这个句子完全正确。意思是“马上要给学生们看看测验是多么难。”

如果句子需要使用被动语态的话，自然要用be about to be done，此用法比较常见。例如：

1. He knew roughly what was about to be said.

他大概知道要说些什么。

来自柯林斯例句

2. He was under pressure to resign and was about to be sidelined.

他承受着被迫辞职的压力，很快就要被排挤出去了。

来自柯林斯例句

3. The water boards are about to be privatized.

水务委员会将实行私有化。

来自柯林斯例句

4. It's about to be cut loose from the state on which it has so long depended.

它将脱离长期依附的国家，实现独立自主。

来自柯林斯例句

5. The British are about to be given a sharp reminder of what fighting abroad really means.

有些事将很快让英国人清楚意识到海外作战究竟意味着什么。

来自柯林斯例句

6. Argentina has lifted all restrictions on trade and visas are about to be abolished.

阿根廷已经取消所有贸易限制，签证制也将废止。

来自柯林斯例句

7. An agreement was about to be reached when the other side raised new issues.

快要达成协议的时候,对方又节外生枝。

来自《现代汉英综合大词典》

8. Hardly had he returned to London than an anonymous well-wisher called to say he was about to be raided by Customs & Excise.

他刚回到伦敦，就有一个不愿透露姓名的好心人打来电话说，他即将面临海关的突击检查。

来自柯林斯例句

9. She is about to be crowned Empress again. 

她又将被册封为皇后了。

10. It's too late, the film is about to be finished. 

太晚了,电影就要结束了。