The National Citizen Service generated eight million extra volunteering hours in 2013 and 2014, an independent evaluation of the scheme has found.
The Cabinet Office-funded initiative offers a residential programme and volunteering activities for young people aged between 15 and 17. Last year the NCS Trust, which manages the service on behalf of the government, ran programmes in the summer, autumn and spring.
The market research firm Ipsos Mori, which wrote two reports evaluating the 2014 programme and the 2013 programme’s impact a year after its completion, found that NCS graduates volunteered for an additional six hours a month on top of the 30 hours of social action they completed as part of the NCS programme – even one year after finishing the course.
But the evaluation found that the expected impact on educational participation had "not emerged in the manner originally anticipated".
The NCS has struggled to meet government participation targets in recent years: a Cabinet Office report in October found that the programme had more than 57,000 participants in 2014, against a target of 80,000.
A cost-benefit analysis carried out during the Ipson Mori evaluation found the 2013 summer programme had between £1.25 and £4.65 of benefits per £1 spent, and between £0.78 and £4.70 per £1 for the 2013 autumn programme.
The 2014 NCS delivered social benefits of between £4.4m and £18.3m at a cost of £5.9m for the spring programme, between £70.8m and £252.6m at a cost of £63.4m for the summer and between £14.3m and £25.4m at a cost of £14.9m for the autumn, evaluators found.
The evaluation said the programme was successfully making teenagers more capable, confident, connected and compassionate.
An NCS Trust spokeswoman said that because the evaluation showed the summer programme was the most effective, the NCS would focus growth on this programme in future.
But it would keep the non-summer programmes "so that young people who cannot commit to a four-week summer programme for family, work or other reasons do not miss out on this life-changing experience", she said.
In its comprehensive spending review last month, the government announced that it would increase the number of places on the scheme from the current 80,000 to 300,000 a year by 2019/20 at a cost of £1.1bn.
Michael Lynas, chief executive of the NCS Trust, said: "These evaluations show that NCS is much more than an enjoyable summer experience – it is making a long-term difference and helping to build a more socially cohesive, mobile and engaged country.
"More than 300 charities bring NCS to life, and they should feel proud that they have enabled more than 200,000 teenagers to fast-forward their futures, and to give an extra eight million hours to their local communities."