The rollout of Covid vaccines to healthy 12 to 15-year-olds has begun in England and Scotland.
Invites are also being sent out in Wales this week, while in Northern Ireland the teen vaccinations are likely to be offered from October.
It is part of the drive to get more people protected ahead of winter as the pandemic continues.
Booster shots are also being offered to over 50s, vulnerable people and health and social care workers.
Jack Lane, 14, who received his vaccine in Leigh-On-Sea, Essex, said: "I am proud to have had my vaccination so that I can remain in school and continue in my education - the jab was quick, easy, and painless."
Experts say that unlike for older people, there is only a marginal benefit to vaccinating adolescents on health grounds alone given their risk of becoming seriously ill with Covid is small.
And although vaccinating secondary school-age children won't stop the virus circulating, it could help keep more pupils in class.
Jack's mother and assistant principal at his school, Michelle Lane, said: "I'm really proud of him and I'm really relieved that he's had it done.
"I really believe that the students need to be in education. There's been a lot of issues around mental health with the lockdown and home-schooling and by vaccinating the students we can keep them in the academy and give them what they need, so I'm really pleased for him and all of the students."
Some three million children are eligible for the jab across the UK after the chief medical officers of the four nations advised younger teenagers should be offered a single dose of the Pfizer vaccine.
Parental consent will not be needed if a child is considered competent to make a decision by themselves, weighing the benefits and risks, including the small chance of a very rare side effect from the vaccine.
Dr Nikki Kanani, GP and deputy lead for the NHS Covid-19 vaccination programme, said: "The vaccine is safe and effective and I would urge families to work closely with their schools based vaccination team to get their loved ones vaccinated when they are invited to protect themselves and their families ahead of the winter period."