A schoolboy wore a stab-proof vest beneath his uniform after being threatened.
The boy, 16, is believed to have worn it on his journey to and from the Lammas School in Leyton, east London.
A police officer was made aware of a pupil wearing "what was believed to be a stab-proof vest", said a spokesman for Scotland Yard.
He added that in consultation with the school "the officer told the pupil to remove the vest".
Police became aware of the vest through the local Safer Schools officer - part of a programme running in 34 local education authorities, with high levels of truancy and crime.
They have become increasingly concerned about the number of people arming themselves with knives - nearly 80 people were killed in knife attacks in London last year.
They have also been running several campaigns in an attempt to alert youngsters to the dangers of carrying knives.