I don't know if it's a "bad idea" but it does sound risky! I suppose it depends how easy / catchy the song is? The worry is that Y7s can be a funny lot and very self -conscious about singing out loud! You say you're planning to use instruments well? Therefore why not just do the song and work out an arrangement of some sort that the children could use to accompany the song? Alternatively as it's always good to work towards a 'performance' at the end of the lesson, split them into groups and give the pupils a choice- any of them who want to sing can - any who want to play instruments instead can do that too. When it comes down to it though, for an interview I'd personally only do something that I'd done successfully before - atleast several times over. Then you'll feel confident and the children will feed off that confidence. Don't know if that helps or not, but whatever you decide to do, hope your interview goes well.