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In a brief letter sent to the school community on Wednesday, the school’s board of trustees chairman Glenn Moors said Mr Caldwell had been the principal at the school for eight years, during which time he led "a cohesive team with our students achieving great outcomes".
He said Mr Caldwell had decided to resign.
A farewell would be held at a date to be confirmed and the board, pupils and staff wished him all the best.
It is understood Mr Caldwell had been on leave for a time before his resignation.
Mr Moors said he could not answer questions about the resignation as details of employment relationships were confidential.