Are the pens and tools taken from female students that remained in the teacher's possession after the end of the year considered unlawful (haram), and what should be done to return them? And is the teacher sinful for not returning a personal tool belonging to a female student, which she requested to make a teaching aid, and then the student forgot to take it after the lesson ended?

The questioner's sister was not correct in taking the students' pens, and there is no legal basis for it. This is because financial punishment (ta'zir bi al-mal) can only be imposed by the ruler (wali al-amr) or his deputy. The four madhhabs (schools of thought) are in agreement on the prohibition of financial punishment. She must seek Allah's forgiveness. If it is possible to return the pens to their owners, she should do so; otherwise, she should donate them to the needy, and the reward will be for their owners.