About 125,000 of London's secondary school children will receive a free ticket to the Olympics in 2012.
Organisers will pay for 50,000 pupils aged 10 to 18 to go to the Games, while Mayor Boris Johnson is promising to find funding for the others.
But tickets "will have to be earned", said Olympics minister Hugh Robertson.
Schools joining London 2012's Get Set education network and an Olympic-style school sports competition will be "at the front of the queue", he added.
Mr Johnson is hoping to find a commercial sponsor to cover the cost of his share of 75,000 tickets.
Otherwise the funding will come from the £625m contribution to the Olympics being made by council taxpayers in London.
But the scheme was criticised by the leader of Greenwich Council, Chris Roberts, who said a ticket should go to every child in his borough and the other local authority areas which are hosting the Games.
"Olympic organisers have had five years to plan a ticket allocation which would permit the children of the host boroughs to attend the Games taking place on their own doorsteps.
"Instead, seven out of eight schoolchildren will be denied that chance.
"The children of the East End were used by the Olympic organisers to win the bid for the Games in Singapore; they have been called upon endlessly to take part in photo opportunities to build support for the Games."
On Friday organisers revealed the price structure for tickets, with the entry to the opening ceremony costing between £20.12 and £2,012, and a seat at the men's 100m final priced at up to £725.
Three-quarters of the 8.8 million tickets available will go on sale in March.