As to the deputy head's given reason (a lesson about safety and guarding against theft), I'd tend to think that was his genuine reason. Theft of valuable personal items in a school is a major headache for the school. The police may get involved. The school doesn't want weaker-minded students being constantly exposed to the temptation, some succumbing to that temptation, and some getting caught and having to be expelled, etc. Better to make students aware of the risk and to have rules against students bringing valuable items to school.
As to the deputy head's given reason (a lesson about safety and guarding against theft), I'd tend to think that was his genuine reason. Theft of valuable personal items in a school is a major headache for the school. The police may get involved. The school doesn't want weaker-minded students being constantly exposed to the temptation, some succumbing to that temptation, and some getting caught and having to be expelled, etc. Better to make students aware of the risk and to have rules against students bringing valuable items to school.