The Swan School in Oxford has already been criticised for the move by a disgruntled parent whose daughter complained she had been 'left hungry' after her first day.
A few meat and fish options are available for snacks at break time - but it's strictly vegetarian in the canteen over lunch.
The mum said online: I've never known a school to be like this. I've heard that a lot of children were very hungry as well - there should be option at dinner time [to] eat meat."
The Swan School, which made parents aware of the vegetarian policy prior to admission, opened to its first cohort of about 100 pupils on Monday and will teach 1,260 pupils when it is full.
Options on the menu include lentil lasagne, minted pea and feta frittata and Quorn sausage and potato bake.
Kay Wood, headteacher of The Swan School, stressed the decision to provide vegetarian meals isn't a judgement of those who do eat meat, and said: "We are by no means the first school to do this and I believe that if we fast-forward ten years then many, many more schools will be just serving vegetarian meals.
"We have chosen to serve vegetarian meals at lunchtimes for three main reasons. Firstly, it allows us to serve better quality meals for the same money. Secondly, there are huge environmental and sustainability benefits. Most importantly, it allows students of all faiths and different dietary requirements to eat together."
Ms Wood added: "Packed lunches are not banned because the students might bring in meat - we don't allow them because we want all the children and staff to eat together and engage at lunchtime.
"It goes without saying that we are committed to making sure students are not hungry - we offer free breakfast every day for all students."
Pupils can get free porridge in the morning and can buy healthy snacks at break time, but they are not allowed to bring in any other food.
Opposing academic research has been published recently about the health impact of vegan and vegetarian diets.
Ms Wood added that she is hoping this move will encourage pupils to consider their surroundings: "The decision certainly is a way of making our students think about the world more than they might otherwise - and that's what schools are for.
"Everyone can see that young people are becoming moved to action on environmental issues, and the initial impression is that our students have embraced the concept of vegetarian meals."
However, prominent environmental campaigners and celebrity chef Jamie Oliver have praised schools for shunning meat.
Speaking last week about his new show, Meat-Free Meals, Mr Oliver said: "The idea of schools going veggie is a brilliant idea - it would save loads of money."