今日 有個女同學比人打 要全班交文章 但係當時我在擦黑板 點寫

既然你在擦黑板, 就照直寫出來囉. 如果你冇親眼見到, 就唔好人云亦
云. 唔需要問其他人, 老師問起的話, 就講你當時在擦黑板, 和聽到甚麼,
就可以了.

Tell the teacher know u don't really know what had happened. That will be enough.
But remember, DON'T LIE.