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 (SNBTS) visited Holyrood Secondary School in Glasgow today 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. SNBTS wants to make teenagers more aware that they can start to donate blood at the age of 17 and continue to do so for life.
Sammi and Rachel joined nurses at the school to thank pupils for getting involved in the campaign.
Sammi, a pupil at Beath High School in Fife, said: "I went through about two years of treatment for leukaemia and throughout that time I had about 20 units of blood given to me so it's really special being here and seeing people donating and being generous.
"When I was in hospital I would see people before and after they received blood and the difference is amazing. Before it most people are lethargic and pale but then after it they can be running up and down the ward with rosy cheeks."
Rachel was diagnosed at 17 and her experience encouraged her friends and family to become donors.
"I really, really relied on blood donations when I was being treated," the 19-year-old from Edinburgh said.
"It saves your life, without it I wouldn't have been able to complete my treatment. Anybody at any time could need blood so it's nice to see so many young people coming forward to donate and hopefully they will continue to do it.
"When I was ill a lot of my friends went down to a local blood drive to donate and I went along and cheered them on so that was amazing."
So far this year 316 17-year-olds have given their first blood donation. In total, more than 2,000 new donors have signed up since January 1.
The SNBTS is to visit a host of schools across the country during the campaign drive but other young people can find out the location of donation centres online.
Dr Moira Carter, head of donor services at the SNBTS, said: "Bloodstocks in Scotland are very healthy at the moment and we want to make sure they stay healthy in the future, and to do that we're asking young people to start donating early and keep on giving blood throughout their lives.
"What we have noticed in the last 10 to 15 years is that the average age of donors is going up and it's now over 40 so we want young blood into the system.
"Sammi and Rachel, the two lovely young girls, would not be here today if it wasn't for blood donors. When you're young you don't think it's going to be your friends, family or even yourself that might one day need blood.
"When most people turn 17 they know that they can get a driving licence but they can also get their licence to save lives through blood donation and we want to get more young people signed up as donors."
People can register as a donor and find out more about the campaign at www.scotblood.co.uk.
Watch the video by David McCaffrey about the campaign below -