Millions of pounds appear to have been paid out in taxi fares ferrying teenagers between lessons in schools and colleges over the past financial year.
Figures obtained by TES Cymru reveal Merthyr Tydfil, the smallest local authority, has spent pound;60,000 transporting pupils to the town's college from four secondary schools.
One north Wales school has also spent more than pound;15,000 on taxis since last April.
But if every secondary has spent a similar amount, the bill across Wales is already well over pound;3 million and likely to soar if alternative solutions are not found.
Private hire taxi firms have been profiting from the phasing in of the skills-led 14-19 learning pathways, a government policy aimed at offering more vocational choice to teens.
All schools and colleges will be required to work together to provide a minimum of 30 courses to their pupils under common timetables by 2012.
Schools are reimbursed by local authorities for fares, but teaching unions say the cost is too extravagant in the current financial climate. Heads have repeatedly accused the Assembly government of failing to consider the cost and practicality of transport.
Last year, the government backed down on plans to introduce a new measure making the 14-19 pathways statutory by September.
Gareth Isfryn Hughes, head of Ysgol Tryfan in Bangor, says his post-14 pupils now have a greater choice of courses by travelling to nearby Coleg Menai and secondaries in Caernarfon and Bethesda.
But he says: "The cost of taxis is particularly expensive and it's a great concern of mine. I agree with the measure to make the 14-19 pathways statutory, but where will the funding come as more courses are offered?"
Officials say high transport costs are the most cost-effective way to give learners more choice, which will benefit the economy in the long term.
Teresa Winiarski, of Merthyr council's integrated children's department, believes the borough will be paid back with the long-term advantages of better qualified young people.
From September, the council is planning a centralised system of regular shuttle buses between the schools and Merthyr Tydfil College.
But even in advanced 14-19 networks, such as Caerphilly, travel is proving to be a major cost.
Originally, schools paid for their own taxis up-front, but now the council centrally funds a system of regular taxi and bus services.
After it expanded its post-14 options and introduced common post-16 timetables in 2007, Caerphilly saw an almost 30 per cent rise in commuting pupils.
But over the last financial year transport costs shot up by 44 per cent to pound;111,364 - the equivalent of pound;3.50 per student, per week.
Dr Philip Dixon of the Association of Teachers and Lecturers Cymru said learning pathways must not become a "bonanza" for taxi and bus firms, and said the cash should be kept for education.
Deputy minister for skills, John Griffiths, has already said local area networks will have the flexibility to work out their own transport arrangements, but he wants travel kept to a minimum.
Although local authorities can get Assembly government funding for transport, officials have said they will consider applications on a case- by-case basis to see if alternatives such as virtual learning or video- conferencing are more suitable. Another solution being considered is for teachers and lecturers to travel between colleges and schools instead.
The Assembly government did not respond before deadline.