A survey by Elwa, the Welsh post-16 education funding quango, says a quarter of school sixth-form students are turned off because they do not have enough courses to choose from.
It says more information should be offered to 16-year-olds about options available in colleges so they can make the right choices.
John Graystone, chief executive of fforwm, the association of Welsh colleges, said: "This survey indicates that the current system is not meeting the needs of many young people.
"School sixth forms are often not in a position to offer a wide variety of courses, so young people settle for a course that doesn't quite match their aspirations.
"The obvious way to extend choice to students would be through collaboration with colleges, which are equipped with state-of-the-art facilities and have a wide range of expertise, allowing them to offer a more diverse curriculum. There are already some excellent examples that involve co-ordinated timetabling between school sixth forms and a local college."
He believes such collaboration could allow students to be released for vocational tuition while continuing the bulk of their studies at school.
The study also showed that many school sixth-formers had not sought advice about where to study.
Elwa is planning a series of activities in the spring to promote greater collaboration.