you can't imagine that a top student should have failed in the exam.这里为什么用 should?

you can't imagine that a top student should have failed in the exam.这里为什么用 should?
you can't imagine that a top student should have failed in the exam.
这里为什么用 should?

you can't imagine that a top student should have failed in the exam.这里为什么用 should?
因为本来这个结果就跟另人惊讶,以至于别人都不敢相信是真的.所以这边有虚拟来表述那种感情.

虚拟语气。所以用should
would也可以

你简直无法想像，成绩最好的学生居然这次考试不及格。

should have done是对过去的猜测

should have done 是一个固定的用法，表示应该做而实际未做；反之 should not have done (shouldn't have done) 表示不应该做的事情，但实际上已经做了。

should可用于表示惊奇、感叹、不满等感情色彩的句子，如指过去的动作，则要用should have done sth. Would 和 may通常不带感情色彩。
如同: I’m surprised that he _____ in the exam
要用should have failed