Saying I have fever is 熱があります. But my teacher messaged me that there won't be any class today because apparently 先生の女の子は熱が痛いです. Isn't saying 熱が痛いです is wrong because "fever" cannot feel pain?
Yes, 熱が痛いです is wrong. You can say 熱があります, 熱が出ています or 熱を出しました.
Or your teacher might possibly have wanted to say 熱で頭が痛いです.
One more thing, 先生の女の子 is not natural if it refers to his/her daughter. 先生の娘 is fine.
If I can still go on, the particle は should not be used here, since the topic is not the daughter's fever but the reason of cancellation.
I would say: