A teacher at a prestigious school in Sydney's inner west has been charged with online grooming offences with police alleging he sent lewd images to someone he thought was a teenage girl.

Police will allege 39-year-old Alexander Simpson, who teachers English at Trinity Grammar School, sent sexually explicit images to a 13-year-old girl online.

However the teen was actually an undercover police officer.

Detectives allege they started talking to the Mr Simpson, from Sydney's southwest, under the guise of the teenage girl earlier this month.

Police will allege he also engaged in these conversations on his mobile phone while at work.

The school teacher was arrested outside the school at Summer Hill about 3pm on Thursday before officers searched his Padstow Heights home where they seized clothing and electronic devices which will undergo further examination.

He was taken to Burwood police station and charged with using a carriage service to send indecent material to a person under 16, and using a carriage service to solicit child abuse material.

Mr Simpson spent the night behind bars after he was refused bail and will appear before Burwood Local Court on Friday.

He has worked at the prestigious school since 2015.

Originally published as Elite school teacher charged over sexting