Headteachers have voiced worries that new rules designed to improve pupil behaviour on transport to and from school could lead to litigation from parents if their children are hurt.
The Assembly government's travel behaviour code, which came into force this month, aims to stamp out unruly behaviour and improve safety among pupils on journeys between home and school or college.
It covers all forms of transport, including bus, train and car journeys, as well as walking and cycling, for all learners aged five to 19.
Schools and local authorities will have the power to act against pupils who misbehave and for the first time heads will be able to impose sanctions, including exclusion for the most serious incidents.
But while the code has been widely welcomed among the profession, a number of heads have told TES Cymru they are "uneasy" with the implications of the government's statutory guidance.
They fear that it places an "unreasonable" responsibility on them to deal with incidents outside their control.
Headteachers' unions have asked for clarity on the guidance, which states that the code is part of a school's behaviour policy, and that heads must ensure pupils at their school comply with it.
Gareth Jones, secretary of the Association of School and College Leaders (ASCL) Cymru, said: "I don't think the implications of this have been fully thought through.
"For example, if there's an incident of bullying on the walk home from school, the head could be exposed to claims of negligence if he or she is deemed to have failed to intervene to prevent it.
"This is new territory and no one it sure where it could lead."
Anna Brychan, director of the National Association of Head Teachers Cymru, said: "It is not reasonable to expect heads to be held to account for everything that happens on a pupil's journey to school.
"There are concerns about the ultimate outcome of a legal claim made over the code and exactly what a head's responsibilities are."
But an Assembly government spokesman said heads were already able to deal with incidents that happened outside school. However, these powers have not been clearly defined until now.
The spokesman said the new guidance would not place significant new burdens on schools.
"If there is a breach of the code, the headteacher is able to take disciplinary action against a pupil if that is appropriate, but there is no duty on the headteacher to take such action," he said.
The government expects each school to have a nominated member of staff to deal with travel behaviour, and to work closely with transport providers and local authorities to ensure incidents are dealt with appropriately.