词语之间的修饰关系，应根据语义逻辑判断。介词短语in part 修饰后面的on their performance in the group，而不是相反。

1. on their performance in the group：依据他们在小组的表现。修饰谓语动词grade，作方式状语。on = based on 根据；依据。

2. in part：名词词组，副词用法，等于partly，意为“在一定程度上；部分地；不完全地”。修饰后面的 on their performance in the group。

Business schools grade their students partly on their performance in the group.

【翻译】商学院在一定程度上是根据学生在小组中的表现给他们打分的。