Papal visit: Pope Benedict XVI focuses on education
The Pope has led an assembly of children at a Catholic college in west London on day two of his state visit.
Earlier, in an apparent reference to the Church's child abuse scandal, Benedict XVI said the safety of children was vital in all schools.
In his address to around 4,000 pupils in Twickenham, he warned against the limitations of celebrity and science.
He will later make a speech at Westminster Hall and hold joint prayers with the Archbishop of Canterbury.
At a gathering of religious congregations ahead of the assembly, the pontiff said: "I wish to add a particular word of appreciation for those whose task it is to ensure that our schools provide a safe environment for children and young people."
He said the life of faith can "only be effectively nurtured when the prevailing atmosphere is one of respectful and affectionate trust".
He also spoke of his "deep appreciation" for those who devoted their lives to teaching and paid tribute to the "outstanding contribution" made by religious orders to education.
The assembly included an official inauguration of the John Paul II Institute for Sport, an athletics complex in the college grounds.
Others to take the stage included Blue Peter presenter Andy Akinwolere and former Manchester United star Brian Kidd.
The pontiff told the crowd young people were often encouraged to model themselves on celebrities.
He said: "My question for you is this: What are the qualities you see in others that you would most like to have yourselves? What kind of person would you really like to be?'
"I'm asking you not to be content with second-best."
He went on to say having money or a successful career was not enough to make people happy, but true happiness could be found in God.
Tight security surrounded the event, called The Big Assembly.
It took place as the Metropolitan Police revealed five men had been arrested by counter-terrorism officers in central London.
Monks and nuns had waited in long queues and were frisked by police.
Around 100 protesters against the Vatican's record on gay rights, equality and birth control had gathered ahead of his arrival, amid tight security.
The Church sees it as an opportunity to celebrate the work of more than 2,000 Catholic schools across the UK, in partnership with the state.
But critics say it could fuel hostility to faith schools and serve as a painful reminder of the child abuse scandal within the Catholic Church.
There has been a wave of cases in which Church authorities failed to deal properly with priests accused of child abuse, sometimes just moving them to new parishes.
Lord Patten, who has helped the government co-ordinate the visit, said the Catholic Church was dealing with the crisis.
He said: "For many of us, one of the worst aspects of all this was that the Catholic Church tried to hide what was going on.
"Well, I think everybody has learnt from that and learnt that institutional arrogance, the way institutions behave, can sometimes get tragically in the way of their message."
The Pope later met representatives of other faiths, including the Chief Rabbi Lord Sacks, and will go on to visit Archbishop of Canterbury Rowan Williams at Lambeth Palace.
The meeting is a gesture of reconciliation on both sides, as Catholic archbishops lived at the palace until England, under Henry VIII, broke with Rome.
The Pope will then make the most political speech of his visit, to members of both houses of Parliament, at Westminster Hall, our correspondent adds.
He is likely to stress the value of Catholic social teaching and link it with ideas of community-building contained in David Cameron's concept of the Big Society.
The day will end with the Pope celebrating Evening Prayer at Westminster Abbey, before a state banquet is held in his honour, although he is not expected to attend.
The trip is the first to the UK by a pontiff since John Paul II in 1982.
It is also the first to be designated as a state visit because the Pope was invited by the Queen rather than the Church.
Dioceses in England and Wales have reported thousands of unfilled places for a vigil in London's Hyde Park on Saturday and a beatification Mass in Birmingham on Sunday for 19th Century cardinal John Henry Newman.