Question

The students are cleaning the classroom改为被动语态

Answer 1

原句的意思是：学生们正在打扫教室。
改成被动句是： 教室已经被学生们打扫干净了。
被动句的用法是：Be+动词的过去分时。
因为是已经被打扫干净了，所以be用过去式，同时因为教室做主语，be动词用第三人称单数形式，所以是用was.
整句话是：The classroom was cleaned by the students.

Answer 2

the classroom is being cleaned by the students. 应该是现在进行时的被动语态， 不建议改为过去式。

Answer 3

　　The classroom is being cleaned by the students.

Answer 4

The classroom is being cleaned by the students.