More than 1,800 pupils at a city secondary school were ordered to shelter under their desks this morning after an intruder was seen leaping the gates.
St Paul's school at Leadenhall was in official lockdown while police scoured the premises.
Terrified students texted their families the news at around 11.30am.
But the scare turned out to be unfounded - although police praised the school for following its emergency protocol.
A St Paul's spokesman said a male was seen scaling the entrance gates and this triggered the lockdown drill.
But it turned out to simply be a student who had forgotten his lanyard, she said.
"All was well and the police were very impressed with how well we followed the procedure... Part of that procedure is for students to go under their desks," she said.
No arrests were made and the school is now operating as normal.
One relative of a student said: "It was all really worrying. My sister texted me saying she was hiding under her desk because there was an intruder in the school and he could be armed."