you can't imagine that a top student should have failed in the exam.

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.

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

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。

虚拟语气。所以用should

would也可以

should have done是对过去的猜测