Some children staying in hospital are not getting as much of an education as those in England, a study has found.
Children's Commissioner Rocio Cifuentes said there were also "unequal experiences" for children across Wales and called for clearer guidance.
One teacher said a pupil asked to study for her maths GCSE on the day of her terminal cancer diagnosis.
The Welsh government said all children, regardless of their circumstances, had a right to a full education.
Isabel Thomas, a teacher at a children's hospital in Cardiff, said the girl, who went on to achieve several GCSEs, was "probably the bravest young person I've ever met".
She said learning maths "broke up the day for her. It gave her a sense of normality... which is what she was craving in a very traumatic time".
What education is provided in Welsh hospitals?
In Wales, lessons are offered to children aged four to 16 who have been in hospital for more than 15 days.
However there is no requirement for how much must be offered, whereas in England it must be full-time education when in the best interest of the child.
The commissioner's report found the amount offered by each local authority differed across Wales, varying from five to 20 hours a week.
Ms Cifuentes said this meant differences for children across the country.
She said councils should be able to provide the equivalent of a full-time education, as is offered to children across the border.
"They should be entitled to exactly the same educational offer as any child, the fact that they are not well and having to be in hospital... shouldn't reduce their entitlements to education", she said.
The report highlights differences between local authorities in the way teaching is funded. For example, some hospitals recoup costs from the child's school.
One parent in the report described the stress of being told by their child's school it had received a bill for lessons the child had received in hospital.
Another parent, Tim Jones, whose 17-year-old son Ellis has spent much of 2023 in hospital, said education was part of a "jigsaw" of therapies that helps take the pressure off parents, children and schools.
Ellis, who was diagnosed with muscle condition dystonia when he was 13, now struggles to talk, eat and walk, said Tim, a former teacher himself.
Tim now volunteers in the children's hospital by making Lego creations with the children - something he did with Ellis during his long stays in hospital.
"Hospitals can be an alternative universe sometimes - you're eating at different times, sleeping at different times," he said.
"It's not like being at home and if the child is still having a kind of routine and some kind of contact it just keeps that in the back of the brain."
The Welsh government said it was publishing new guidance for local authorities on provision for children receiving education other than at school "including children who are unable to attend school due to being a hospital inpatient".