Elizabeth Bruce admitted taking cash, jewellery and other items totalling £5,500 at St John's College in Southsea in a spate of incidents prosecutors say occurred between July 2014 and March 2016.
Speaking in court, the school's bursar Simon Merriam said: "This set of incidents has had a huge impact on the boarding community at St John's. It led to boarders not trusting each other."
Thefts by Ms Bruce were brought to an end when a covert camera captured her searching through a purse planted by detectives.
At Portsmouth Crown Court, she was given an eight-month prison sentence suspended for 15 months, told to pay nearly £850 in compensation, and she was instructed to complete 30 days of rehabilitation and 120 hours of unpaid work.
Simon Merriam added: "We're 25 boarders down as things stand and whilst this is not all directly related to the thefts, we know they have contributed. Our reputation overseas has very much been damaged."
Founded by a Lasallian community of Brothers established by John Baptist De La Salle, the patron saint of teachers, St John's College was founded in 1908.