Primary school children with cancer are being bullied, losing friends and missing out on their education, a survey by a cancer charity suggests.
Clic Sargent said its interviews with more than 200 families raised concerns about how schools supported pupils who had been diagnosed.
It showed the side effects of treatment, such as hair loss and weight gain, were a source of bullying.
There were also reports of delays in organising home tutoring.
About 1,600 childhood cancers are detected in the UK each year. Leukaemia, a cancer of the white blood cells that fight infection, is the most common childhood cancer.
Josh Hill, from Cheshire, was diagnosed with leukaemia in his first week at primary school and missed half of that year.
His parents had concerns about his levels of basic reading and writing.
His mum, Lynda, felt he was "falling behind" at school and never received the support to help him catch up.
"I felt like the one who was always asking for work from school, it wasn't the other way round," she said.
There were also problems with bullying.
His mother said: "In primary school, a lot of children were really understanding, but there were a few kids that would pinch the hat he wore to hide his bald head.
"They did it on purpose and it must have really got to him."
The survey showed that more than a third of parents said they were unhappy with how their school had helped their child get back into the swing of school or to catch up with missed work.
Nearly half said their child had grown apart from schoolmates and some had lost friends.
The charity's chief executive, Lorraine Clifton, said many children received the support they needed but some were still being let down after being diagnosed with cancer.
She said: "No child should have to miss out on their education because they've had cancer - and it's distressing to hear that some are teased and even bullied on their return to school.
"Sometimes parents, already struggling to cope with their child's diagnosis, have to fight to get the help their child needs - and they can feel really let down by the system."
A Department for Education representative said: "Bullying in all its forms is completely unacceptable, and is particularly deplorable when aimed at a child with a serious illness.
"Schools should be safe for all children to learn and reach their full potential free from fear."