How to organize a small inter-house table tennis tournament at school?

My head of house has asked me to put together a casual lunchtime table tennis competition between people in my house at school. There are 5 homerooms in my house who the competition will be between, each with around 30 kids ranging from year 8 to year 12. I'm thinking of trying to get people to meet other people in different grades, so therefore have different grades verse eachother. Any ideas?