Nathan Davis, 22, of Bournemouth, Dorset, was serving in Iraq in the 2nd Battalion, The Royal Welsh, in 2007 when an eyesight problem forced him to leave his job.
After joining the SkillForce internship programme for injured and long-term sick military personnel, he became a full-time instructor for the charity.
SkillForce works with 13 to 19-year-olds to help them gain qualifications and improve their prospects of further education or employment before they leave school.
The teenagers spend one day a week with an instructor for up to two years - potentially spending up to 20% of their schooling with SkillForce.
Mr Davis uses his military experience of relationship building and problem solving to help inspire the teenagers and offer them alternative ways of learning.
He said: "With the students we work with, I'm quite close to their age so I can relate to them but having been in the military definitely helps with the element of respect.
"It was always an interest to teach but I had never seen myself as doing something like this full-time."
According to SkillForce, 35,000 young people leave school each year without qualifications
Mr Davis is currently working with Year 11 pupils towards an employability qualification before they leave school in a few months.
He said: "With the time we spend over the two years we really get to know those people and work with them. I'm definitely interested to see where they go from here."
SkillForce works in 150 schools across England and Wales. The majority of its mentors - about 75% - come from military backgrounds.
David Hubbard, SkillForce's team leader for the Bournemouth and Solent area, said: "What the military personnel bring to the schools is life experience.
"The instructors vary in terms of age but, if you think, a lad in his 20s who has perhaps spent six years in the Army has had a huge experience of life that would perhaps take other people years to accumulate.
"Their training and teamwork are fundamental to everything.
"They build a trusting bond - a lot of youngsters are disengaged with schooling or need confidence and this is how the instructors can help - by helping build that self-confidence."
According to SkillForce, 35,000 young people leave school each year without qualifications.
The charity said those young people are four times more likely to end up not in education, employment or training than those with qualifications.
It said 93% of its graduates gain a recognised qualification and 60% go on to further education.