A former teacher persisted with a sexual conversation for more than 40 minutes after being told the other person was school-aged.

But William Roberto Gulson told a court on Wednesday said he never believed he was messaging a child because their online profile on the adult site stated they were in their 20s.

Gulson taught at Sydney's elite Knox Grammar School and conducted private tutoring before he was charged over his messaging with a 15-year-old girl, posing as a boy, in September 2024.

The 15-year-old and another teenager were running a fake profile on LGBTQI dating app Grindr with the aim of catching pedophiles, the Downing Centre Local Court has been told.

The fake profile told Gulson they were in year 9 before sending Gulson a photo of an erect penis.

A later message stated thei

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