The first thing is how to pair it. Is this rated or unrated? I'll assume unrated, in which case pair them by grade level.
If you want to increase interest within the school create some flyers and post them wherever you can.
Oh, and if this is your first time running a tournament it may be a good idea to see if you can find someone from the local club who has experience to help out.
I want to organise a chess tournament at my school for an afternoon, making it open for everyone who wants to join, Have you got any pointers of what I should think about when organising or ideas of what would make people interested in it? I've got the local chess club interested and got a few ideas of who would be able to sponsor with prizes.