Fewer teenagers could leave school for college courses under the skills-led 14-19 learning pathways than was planned.
Instead, secondary teachers and lecturers could be expected to commute between schools and colleges.
An Assembly cross-party committee wants research to be commissioned to examine alternatives to pupil travel, including teachers travelling, the use of mobile classrooms and video-conferencing.
The learning and enterprise committee's report this week also said it wants the impact of the pathways on faith schools to be investigated.
The committee has made a series of recommendations after considering proposals in stage one of the unpopular learning and skills measure.
Members agreed with teaching unions that the Assembly government was being "unreasonable" in expecting a new law by the spring, forcing "uneasy" collaboration between schools and colleges by next September.
Heads want to see a delay of at least one year, and at best three. Many see the transportation of pupils as their biggest logistic and health and safety "nightmare", especially in the rural Welsh heartlands.
There was also concern by the committee that exclusion policies in schools and colleges clash.
Jeff Cuthbert AM, the committee chair, said there was support for the new law, but there were problems.
"Given the importance of this legislation, it is vital that implementation is carefully planned and is successful. Our report identifies those areas that need further consideration," he said.
The report was published as schools and colleges clashed over the proposed law.
The Welsh colleges' association, fforwm, urged the Assembly government to stick to its plans for enforcement. In a thinly-veiled attack on teaching unions, it said it should "withstand calls from some quarters for a delay".
The learning and enterprise committee supported the law in the long-term and suggested amendments so that there were no "get out clauses" for schools.
But committee member Andrew RT Davies, the Conservative shadow education minister, called for the law to be halted, warning the Assembly government is risking a "foundation phase mark II".
The report will come as a second blow to the government's plans for passage of the new law by April. Last week, the finance committee attacked officials for not providing a "reliable" cost of the initiative.
By 2012, all schools will have to offer 30 subjects to their pupils to widen choice and engage more teenagers in learning.
The heads' union ASCL Cymru, who gathered in Llandrindod Wells for their annual conference yesterday, published a survey today showing hidden costs to the 14-19 learning pathways, including huge variations in the cost of courses and transport across Wales. Per-pupil tuition fees varied from Pounds 44 to Pounds 840 and transport costs were up to Pounds 360 per pupil per annum, according to the survey of union members.
An Assembly government spokesperson said it will comment after consideration of both reports.
Leader, page 40
Nigel Stacey, page 41.