When staff and pupils returned to the private school after the Christmas holidays they discovered seven treasured oil paintings had disappeared.
Portraits of the former heads of Aldenham school in Elstree had been removed from the wall of the dining hall. The paintings - some of which date back to the early 1800s - have been valued at pound;30,000. They disappeared between December 21 and New Year's Day.
Among the portraits stolen was one of the school's previous head Stephen Borthwick, who left last year.
The picture, commissioned from local artist John Walton, was due to appear n a portrait exhibition in London this year.
His successor, Richard Harman, said: "It is sad and disappointing. The paintings do not have a huge market value, but they are precious to us because they represent something of the school's history. We are hoping and praying they will turn up."
Police are continuing to investigate the theft. Investigating officer DC Marcus Bromley from Hertsmere police said: "We want to hear from anyone who may know the whereabouts of the paintings, or who may be offered them for sale in suspicious circumstances.
"We're urging dealers to be vigilant too, and to contact us immediately if the paintings are spotted anywhere."