Principal Dan Reddiex said the poplars and eucalypts had been there for more than half a century and were beginning to rot from the inside, causing large branches to fall in recent months.
The trees surround the school's main exit, used by King's and Queen's High School pupils to catch buses.
''It's a health and safety issue.
''We've had a couple of massive branches come down. Luckily, they came down on a weekend.
''But if a student or a member of the public had been under it when it fell, then it could have been a potentially fatal outcome.''
Mr Reddiex said he was sad to see the trees go, and the school board of trustees did not take the decision lightly.
''The trees are a huge part of the history of the school, but the safety of the students and the public is paramount.''
The decision to remove the trees was reinforced after the first three trees were cut down by arborists at the weekend and found to be rotting from the inside out, he said.
A cross-section of the logs showed about 60% to 70% of the trunks were rotten, he said.
The remaining four trees would be taken down during the next two weeks.
''It would have been ideal to wait until the school holidays to cut the trees down, but we can't afford to wait any longer.''
The board planned to redevelop the front of the school and plant new trees.
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