Agree with RMA about taking a register. Don't worry about giving them sheet music - the words (on a projector) will do! Start with some stuff in unison, then add some simple harmonies (Jar of Hearts by Christina Perri is a good one for this). Get them to suggest songs and check the lyrics before saying yes. Do you have a separate accompanist? (Perhaps your school management will pay for one if having a choir is so important for the school PR! Or ask an able student-pianist). If not, www.karaoke-version.co.uk has good-quality backing tracks for a few £ - allows you to conduct and actually listen to the singers. It's worth getting to know the students' ranges, esp. the boys, so that you can select repertoire and give them parts that they can sing. Schedule a performance and let them know the date. If they have something to work towards, they're more likely to step up to the mark. Good luck!