Some 12 million schoolchildren across South Africa have sung a specially composed song as part of official celebrations for Nelson Mandela's 94th birthday on Wednesday.The former South African president will spend the day quietly with his family in his village home of Qunu in the Eastern Cape, near where he was born.Mr Mandela met former US President Bill Clinton on Tuesday.He spent 27 years in jail for leading an armed anti-apartheid campaign.Mr Clinton, accompanied by his daughter Chelsea, opened a new library for the No-Moscow Primary School in Qunu, ahead of his meeting with Mr Mandela.He told the children that Madiba - as Mr Mandela is affectionately called in South Africa - believed in the future and not the past."When I think about Mandela I always think about someone committed to the future," Mr Clinton said.'Extraordinary life'US President Barack Obama and his wife, Michelle Obama, have also paid tribute ahead of Mr Mandela's birthday.The US first couple hailed Mr Mandela's "extraordinary life and steadfast commitment to the principles of democracy and reconciliation".The BBC's Milton Nkosi in Qunu says that across South Africa, Mr Mandela's birthday is traditionally celebrated with the poor communities receiving help from those who are more fortunate.Our correspondent says to celebrate the number of years Mr Mandela spent in public life, people are encouraged to spend at least 67 minutes of their time helping those who are less fortunate.The guest list for a private lunch with Mr Mandela has, however, been kept a closely guarded secret.Mr Mandela stood down as South Africa's president in 1999 after serving one term, handing over to Thabo Mbeki.After leaving prison in 1990, he led the African National Congress party to a landslide victory in 1994 - the first time South Africa's black majority was allowed to vote.