A friend asked me if I would drop off her mezuzos at the office of the sofer located next door to the school where I work.  I had them in my bag, and when I stepped out of the classroom the children opened my bag and ruined the mezuzos.

Q: Are the parents of the children who did this liable?  Am I liable to repay my neighbor for the damage caused by my students?

A: Generally, parents are not liable for damage that their children cause, and teachers are also not responsible for damage caused by students.  However, in your situation you are liable for the mezuzos, since it was negligent of you to leave your friend’s mezuzos where your students could reach them. A shomer —  custodian —  may give items he is watching to his children or to members of his household to watch instead of him, since it is understood that he might do so; but if he gives it to his small children he is liable for any damage, since it is negligent to give someone else’s items to a minor, who is considered irresponsible (Rema C.M. 291:21). Even a child who has reached the age of bar mitzvah but is irresponsible may be considered a minor in this matter (Maharashdam 49).

If an unpaid custodian — shomer chinam — leaves the animals he is watching to go into the city at a time when all the other custodians go into the city, he is exempt if something happens to the animals during that time even if they are stolen. The reason is that he behaved the way shepherds normally do (C.M. 291:12). Seemingly, this halachah is a precedent to exempt you from liability, since a teacher may also walk out of the room momentarily. However, the two cases are not parallel. 

In the case of the shepherd the potential damager is not present when he leaves to go into town; but in your case you acted negligently, since the potential damagers, i.e., children who tend to do damage, were already present when you walked out and left them with access to the mezuzos (See Rosh Nedarim 37a d.h., “sachar shomer”). Also, the animal owner knew in advance that the shepherd would leave the animals unguarded for a period of time, but your neighbor didn’t expect you to leave her mezuzos where your students could get hold of them.

A teacher is allowed to go out of the classroom for a few minutes without concern that the students will inflict damage, but it is negligent to leave behind something that he or she is responsible to protect. Even though the teacher may regularly leave his or her own possessions in the classroom with the students, one must realize that when acting as even an unpaid custodian of someone else’s possessions, one must conform with the Torah’s expectation of responsible behavior; if one does not comply with that standard, he is liable for damage that occurs.

However, each case must considered separately as to whether there was reason to be concerned that the children might damage or steal something while the teacher is out of the room. Even if the teacher is liable, the child’s parent may choose to pay the damages and thereby release the teacher from liability. And if when the child becomes an adult he decides that he is morally (lifnim mishuras hadin) obligated to pay for the damage he caused as a minor (C.M. 349 and O.C. 343), the owner must return the money he received from the teacher (Shevet HaLevi 4:224).