Rickmansworth School in Croxley Green has issued a letter to parents after a group of sixth formers were threatened by youths.
A group of young males approached the students on the Metropolitan Line between Croxley Green and Moor Park on January 24.
British Transport Police were called at 3.42pm to a report of “threatening behaviour” at Moor Park station in Northwood.
Officers arrested three suspects, two 17-year-old boys and a 19-year-old man, when a train arrived at Wembley Park station.
In a letter, parents were urged them to talk about their children about their journey to and from school.
It read: “We have been informed that a group of Rickmansworth Sixth Form students were approached and threatened by a group of young males on the metropolitan Line.
“The incident has been reported to the police and we believe that the group of males have been arrested. This is obviously concerning, and we will be liaising further with the police.
“I would be grateful if you please discuss your son/daughter’s journey to and from school. Please encourage them to avoid other groups and any potential conflict.
Should your son/daughter encounter a similar situation, they inform the police first so they can respond as quickly as possible and then inform the school.”
One of the 17-year-olds was arrested on suspicion of conspiring to commit robbery, the second was arrested on suspicion of attempted robbery, and the 19-year-old was arrested on suspicion of attempted robbery.
All three have been released on bail until February 2.