"At first I thought I had made a mistake," he said. "Then I thought the council had made a mistake, and then I realised it really was that bad and I would have to lose staff."
Five full-time teaching posts have gone - two through voluntary redundancies and three through natural wastage. The result will be increased class sizes. Less able students - concentrated in the smaller groups that will have to take up the slack - will be disproportionately affected with some classes increasing from 15 to 25 pupils.
The school, which has a pound;3 million budget, will also have to abandon an alternative curriculum for less motivated Year 10 students who will be put back in mainstream classes.
Mr Forrest says both bright and less able pupils will suffer as a result, with specialised teaching cut and disruption and discipline problems likely to increase. Workload will also increase as staff take bigger classes more often. Morale is low and teachers are "fed up" with how the Government is treating education.
"I am really rather sickened by this," Mr Forrest said. "I never thought I would have to make these sort of changes and decisions."