Primary and secondary schools across the country are invited to host fundraising activities from October 8-12 in a bid to raise £250,000 to mark the charity's 25th anniversary.
ChildLine is hoping to break the Guinness World Record for the most greetings on a single birthday card by inviting every school taking part to send in their birthday wishes to be included in a giant card.
Founder Esther Rantzen said: "When ChildLine was launched back on October 30th 1986, it received 50,000 attempted calls on the first night from desperate young people who, until then, had no way of asking for help.
"Now, 25 years on we're still providing comfort and protection for children and young people who contact us by phone and online, but we need to raise funds to pay for the service.
"Taking part in the ChildLine Big Birthday week is a fantastic way for schools to show their support, helping us to continue our service for another 25 years and more."
To register, visit www.nspcc.org.uk/thebigbirthday