The counsellors - 20 from Westminster council's social services department, and 10 from the Catholic Children's Society - helped classes throughout the school to express their grief, sense of loss, and worry for the future.
Outside school later, pupils told how the counselling sessions, involving writing letters or cards to Philip Lawrence's family, had helped them.
One first-former said: "I wrote to Mr Lawrence's family. I told them I was sad. I told them it should never have happened. I told them not to feel terrible for too long. And I told them not to worry."
The writing therapy - a commonly used psychological technique in situations of severe grief - was carefully supervised.
Many pupils described how some of their friends had "quietly cried" as they tried to express their innermost feelings.
The counselling sessions were only one part of the "special programme" the school governors and staff had prepared for yesterday. The focus was a special mass conducted by the Archbishop of Westminster, Cardinal Basil Hume.
After the mass, he said: "I told the children that it was very important to come today so we prayed together and prayed for Philip Lawrence and share our sorrow."
Praising Mr Lawrence's concern for his pupils and his bravery, Cardinal Hume said: "I reminded pupils of the [New Testament] text 'No greater love hath a person than to lay down his or her life for a friend'. That is what Philip Lawrence did. This was a headmaster who was a friend to all his pupils."
Although Cardinal Hume insisted that "the person who has done this terrible act should be caught and punished", he added: "We have to learn to forgive. That is the hardest thing."
During his sermon at the special mass, the Archbishop told pupils he had a message for the murderer. "My advice to him is to report to police at once. He needs to acknowledge his crime and pay his debt to society."
On their way into school for the start of the day there was a distinct air of unease as pupils passed the growing shrine of flowers that had built up over the weekend.
Asked by the assembled ranks of newspaper reporters and television crews how they felt, one word was often repeated: "Sad".
James, a first-former, said: "This is such a sad day. Mr Lawrence tried to push us and help us. He didn't deserve to die, not like that."
By the end of the day, after a lunch break of what appeared to be normal play and noise from the school playground, pupils tried to express what the special day had meant to them.
Nicholas, ignoring the efforts of teachers and staff to move him away from television crews and reporters, insisted on having his say.
"It was a hard day. I can't remember everything that happened. I wrote some cards and said some prayers. All we know is that this should never have happened - not here."Reuse content