Many felt that the time which it had taken the board to investigate and reach a decision over the accusations made against Anthony Verity, 56, by his secretary, had made it impossible for him to continue.
As they spoke of their dismay at his departure, despite being cleared of any sexual impropriety with Anne Ridley, 38, all 1,400 parents received a two-page letter outlining the reasons for the governors' decision.
The letter, signed by Sir Colin Cole, the board's chairman, reiterates the reason for his retirement given after a meeting on Thursday between the governors and Mr Verity, saying he had allowed "an inappropriate relationship to develop and that this affected the performance of his duties".
Even though Mr Verity strongly disputes that view, he decided to resign from his pounds 100,000-a-year post after an agreement over a financial package, rumoured to be about pounds 500,000, had been struck. Mrs Ridley has also left and received compensation.
The letter says the process to select a successor will begin next week, but rejects any suggestion that the board dragged its feet, and was always aware of the "need to resolve the matter speedily" following Mr Verity's suspension in August over allegations in March.
But one of the most outspoken parents, Sue Macdiarmid, with three boys at the school, questioned the board's actions.
"The more we hear of this, the more it seems like a catalogue of disastrous decisions made by the board from March onwards," she said. "It reveals the inability to deal with a small management problem. The incompetence of the board has now become an injustice."
She also accused the board of moving the goalposts once it became clear that the sexual harassment allegation could not be proven. The board had called his performance into question when that had not been raised when he was first suspended.
Finally, Mrs Macdiarmid called on Sir Colin and the board to resign, a demand echoed by others who wished to remain anonymous. "Boys, parents and staff at Dulwich College deserve better than this," Mrs Macdiarmid said.
Bob Alexander, clerk to the board, declined to comment.