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.
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。
The classroom is being cleaned by the students.
The classroom is being cleaned by the students.
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