Michael Tilt, who was head of PE, also failed to report when a pupil was stabbed on the trip.
He was found guilty of unacceptable professional conduct at a General Teaching Council disciplinary hearing this week in Birmingham.
Pupils from Sunbury Manor school in Middlesex bought combat knives and ball-bearing guns from a local market while in Spain on a sports trip between May 28 and June 3, 2005.
The committee heard that Mr Tilt, who led the trip, told the pupils to hand any weapons straight to him if bought.
He then collected an array of offensive weapons and took them back to the UK in a bag under his seat on the coach, later returning them to the pupils' parents.
He also failed to immediately report an incident in which one of the pupils on the trip was stabbed and required stitches.
In addition, he was accused of failing to support a staff member assaulted by Spanish youths and staff affected by a "happy-slapping" incident.
His colleague Zoe Roach, who accompanied Mr Tilt on the trip, told the hearing: "I expressed my horror when students were found to be buying knives, but Mr Tilt said if they gave them to him he would keep them until they returned to England, then give them back to the pupils.
"I told him students buying knives was not a good idea, but we disagreed about it.
"When we got back he lined the items up on a stage and asked the parents to take them."
Louise Duncan, Sudbury Manor head, said: "The students were clearly given the message that although they were told not to buy these items, doing so would be acceptable if they handed them to Mr Tilt.
Mr Tilt, who was not present at the hearing, will be restored to the teaching register after 15 months.