before classes,some teachers would ask their students to hand in their mobile phone to prevent from being distracted.

此句除了句首单词小写错误外，还有3处错误：

1. “上课前”，应为：Before class，这里 class 是抽象名词，用法同school ，单数不加冠词。

2. mobile phone 改为复数

3. prevent from being distracted 错误，prevent 缺宾语。这个目的状语，如果逻辑主语是teachers， 那么应改为 prevent them from being distracted；如果逻辑主语是students，应改为prevent themselves  from being distracted

为了避免这种歧义，建议改为： prevent distraction.

全句修改为：

Before class, some teachers would ask their students to hand in their mobile phones to prevent distraction.

原 being distracted 是动名词被动式，作from 的宾语，不是现在分词，更不是“进行时”。网友的基本概念错误，进行时是针对句子的谓语动词而言，介词后面的doing 是非谓语动词，不存在进行时的说法。

这段话，估计是学生作文习作或老师的范文。