The study, commissioned by the educational charity ContinYou Cymru at eight Cardiff schools, found gains in GCSE grades, and reading and spelling ages "over and above the expected norm", as well as an improvement in skills such as communication and teamworking.
And the improved performance did not just depend on pupil attendance at traditional homework or revision clubs. Data was examined from six secondary schools and two primaries, and based on pupil performance before and after the extra sessions. GCSE improvements were based on predicted and achieved grades.
The report's author, Dr Adrian Schmit, said: "In every instance, study-support activities gave substantial and measurable benefits to the pupils involved.
"In many cases there is clear evidence that academic attainment is boosted by study support, whether or not it is specifically intended for that purpose."
In the past year, pupils have been involved in fishing, archery, sign language, yoga, film-making, samba, dance and trampolining as well as the traditional academic and sporting activities.
At Llanedeyrn primary, where 27 per cent of pupils have special educational needs, an after-school course emphasising spelling resulted in nearly all Year 6 pupils increasing their spelling age.
At Kitchener primary, where 70 per cent of pupils do not have English as a first language, Y6 pupils acted as reading peers to their Y3 counterparts.
According to the report, all the Y3 pupils improved their reading ages by 12 months.
Willows high school saw pupils who regularly attended more than one study-support activity gain 2.2 more A*-C GCSE grades than predicted, while dance sessions at Cantonian high improved communication and teamworking skills (see left).
Pam Boyd, executive director of ContinYou Cymru, said: "Overall the message is that out-of-school learning opportunities in the broadest sense help pupils to reach their potential. It helps them improve their results and also to develop as citizens of the future."
Dr Schmit's report follows his 2003 study, Measuring the Difference, which looked at the effects of three years of funding from the National Lottery New Opportunities Fund on out-of-hours classes and GCSE results.