I tend to think of religions as man-made constructs used to explain and control the majority of the population by the few (usually men) that have made it to being the favoured few...
That it was easier to control the population if they thought there was a god watching them and that would punish them for bad behaviour, reward for good etc. Also it's a way of explaining the unexplainable - because you don't have to actually know why the river flooded or there was a forest fire, you can just say 'god did it because somebody upset him/pleased him' and so on.
Many religions have incorporated basic tenets of human morality and tried to claim them as their own but they're not really tied into the religion per se, they're a human thing not a religious thing.
They provide a framework for living that was useful back in time (eg by including things like the eating restrictions for Jews, Muslims and others as well as the be good/don't murder people/etc stuff) but these days isn't really necessary. Caveated that obviously if you got rid of the religious stuff you wouldn't get rid of the basic human decency part - as lots of religions tend to imply!
And whilst lots of people do get some sort of comfort and support from being part of a religion, they are also a source of wars, a way of controlling people (particularly women) and a lot of bad things have happened in the name of religions as well as the good side of things.
however I'm not sure how far you'd be allowed to go through that if the school is religious!
I'd also point out that in some countries atheism is seen as being incredibly dangerous and there is even the death penalty for it in some countries. Countries like Saudi Arabia for example you might expect not to be keen on atheism - and whilst they don't particularly like other religions, they would prefer somebody to be of another religion that to have no belief - I guess they seem to think that if too many people started to think like that they'd no longer have the control they currently have. But places like the USA and Ireland also are anti atheists - in the USA there are lots of places where atheists aren't allowed government roles 
for example.
This map is a good one - shows the 13 countries where being an atheist is punishable by death: www.thewire.com/global/2013/12/13-countries-where-atheism-punishable-death/355961/
Another article that shows where atheism is widespread - in the UK they reckon it's about 66% for example... www.washingtonpost.com/news/worldviews/wp/2015/04/14/map-these-are-the-worlds-least-religious-countries/
And then in Bangladesh recently there have been several prominent atheists murdered.
So being atheist is not just an easy option that means you don't have to get up early to go to church on a sunday etc - it can have serious ramifications on your life if you publicly declare yourself to be an atheist in many countries.