Is my teacher responsible if she lost the cellphone she confiscated from me?

Yes, the teacher or the school is responsible to return your phone or pay for it if it is a public school. If you attend a private school, you and your parents may have agreed to any set of rules, even ones that don't agree with law. The teacher probably should not have gotten into this complex area of law by seizing your property. Since she did so, she should have immediately given you a receipt that both of you signed, with a description of the phone and an explanation of how you could retrieve your phone. She should have probably just told you to put the phone away, but maybe she had reason to think you would not do so. This is different from a bailor - bailee situation, where you voluntarily give something to someone for safekeeping, such as in a coat check. In this situation, your phone was seized by an authority and the authority has a duty to keep it safe for you. I am not sure how she could lose the phone before the end of class. Maybe one of your classmates stole it off the teacher's desk.

Yes, your teacher, or the school, should reimburse you for the phone. Once the teacher took possession of it, the teacher was responsible for safe keeping of the phone until it was returned to you.