The auditors said that the proportion of people in this age group who are inactive is expected to have dropped by 10 per cent by November, a key target for Connexions, which is now in its third year.
The proportion has already been cut by 8 per cent in areas where the service had been established longest, says the report.
The report found that the service, which has a pound;450m budget, has recruited fewer than half the number of personal advisers originally intended.
FE Focus revealed last week that nine in 10 English colleges have to fund additional careers advisers for students aged 19 and under.
Sir John Bourn, the auditor general, said that the service had made significant progress in a short time to reduce the proportion of young people not in education, employment or training.
But he added that it needed to do more to "ensure all young people obtain the support they need."
Connexions, part of the Department for Education and Skills, aims to provide advice and guidance to for 13 to 19-year-olds.
The report found the service was well regarded by most of the 16,000 youngsters who had met one of its personal advisers.
It employs 7,700 personal advisers employed, half the number required and only half of these have had Connexions training.
The report said that Connexions is poorly-understood by teachers and showed wide variations in performance among the 47 partnerships nationwide.
Margaret Hodge, minister for children and young people, said that "excellent progress" has been made in a short time as more than 6,000 young people a week now contact a personal adviser.
"Young people, whether or not they are at risk of dropping out of education, are very positive about the new service."
The report said a target should be set for personal advisers to receive training and issue clear information to show how the service is performing around the country. Auditors surveyed 1,000 schools and 150 colleges.