A 17-year-old boy is being questioned by police over a number of social media threats made against schools across Sydney.

St Andrew's Cathedral School in Sydney's CBD was placed in lockdown on Thursday afternoon after the school was the target of threatening messages.

St.Andrew's Cathedral School was placed into lockdown following a social media threat. Credit:Brendan Esposito

The Anglican school was the third in Sydney to be subjected to social media threats this week, after East Hills Boys High School in Panania and Sir Joseph Banks High School in Revesby were placed in "lock-out" by police on Tuesday.

Police said they had spoken to a 17-year-old boy on Thursday night about the alleged threats and he was assisting with their inquiries.

A number of electronic devices had also been seized by police.

The teenager had not been charged at this stage and would undergo a psychological assessment, police said.