They came to a classroom, in the front of which sat a student.

这个非限制性定语从句采用了完全倒装结构。试比较：

A student sat in the front of the classroom.

本句中sat为不及物动词，后接必具性地点状语。当出于某种原因必具性状语前置（即移至句首），且主语不是人称代词时，则必须完全倒装：

In the front of the classroom sat a student.

In the front of the classroom a student sat. (X)

如果主语为人称代词，则不倒装：

In the front of the classroom he sat.

In the front of the classroom sat he. (X)

网友的这个句子中，关系代词指代classroom，作介词的宾语，且介词短语必须前置，故从句主谓倒装。