Only a fifth (20%) of the 1024 children who responded said they felt that they had someone to confide in at school.
The findings come as many children and young people prepare to sit their GCSEs and other exams, which can lead to some pupils feeling stressed and under pressure to perform.
The poll marks the start of a national drive to encourage children and young people to contact ChildLine with any problem - big or small. Posters will be sent to all schools in England, Wales, Northern Ireland, and the Channel Islands with the message that ‘whatever your worry, it's better out than in'.
The campaign will also launch a new look website for children at childline.org.uk that provides more advice about typical problems children worry about and information on what would happen if they get in touch with ChildLine.
‘Feeling embarrassed' is the top fear which stops teenagers from talking to an adult about their problems - picked by two in three (64%) girls and nearly half of boys aged 11 to 16.
‘I can't find the right words' was the most common reason primary school children gave for not telling an adult about their problems - chosen by 51 per cent of girls and 40 per cent of boys aged 7 to 10.
When asked who they would tell about a big personal problem, 87 per cent of children said they would not talk to a teacher. Sadly, three in ten of all children won't tell anybody and keep their problems to themselves.
Head of ChildLine, Sue Minto said: "These findings echo the feelings of the thousands of upset and frightened children who call ChildLine every day.
"We know children can often find it embarrassing and extremely difficult to tell someone if they have a personal problem. They worry no-one will understand their problem. They fear they won't be believed or taken seriously.
"When children can't talk to a teacher, parent or other adults in their life, ChildLine gives them someone to turn to. Our counsellors understand how difficult it is for children to talk about private matters - they let the children open up at their own pace."
"We want children to know that however big or small their problems are, if it matters to them, it matters to ChildLine."
