Sammi Bradley, 13, and Rachel Laird, 19, each received more than 20 units of blood during treatment for leukaemia.
Both girls are now in remission and are supporting an NHS campaign to reduce the average age of donors. Nurses from the Scottish National Blood Transfusion Service visited Holyrood Secondary School in Glasgow to allow 17-year-old pupils to give blood.
There are more than 140,000 registered donors in Scotland with an average age of 40.
The service wants to make teenagers more aware of the need for giving blood.