Question

it is strange that such a thing _______in your school

为什么选择should happen 不选happed?

Answer 1

“It is +形容词/或-ed+that 从句中的should+谓语动词”句型，常见的要接含有虚拟语气主语从句的词有：
形容词：important, necessary, urgent, essential, vital, natural, strange, proper等
动词的-ed形式：arranged, suggested, ordered, required, desired, demanded, proposed等
It is necessary that the program (should) be loaded into the computer. 有必要把程序输入电脑。
It is strange that he should have learned so much in such a short time. 很奇怪他竟然在如此短的时间内学了那么多的东西。
It is desired that we (should) get everything ready by tonight. 希望一切在今晚前准备就绪。
因此，
it is strange that such a thing _______in your school
要选择should happen 。表示感情色彩，“居然”会发生在你们的学校。

Answer 2

在It is + adj + that的宾语从句中，从句用should 加动词原形, 此处的should可以省略.
这类形容词只有少数几个：strange 、necessary、imperative等
所以选择should happen

Answer 3

注意此句的语气：这种事情居然发生在你们学校，太奇怪了。
表达了说话人的强调，因此要使用should，来表现这种语气。