(from Teaching Primary Music page 43).
Online song banks including Sing Up, which was set up as the National Singing Programme, offer a wealth of material; many songs are presented as lyric sheets or backing tracks with and without the parts being sung, and also as slowed down versions of songs, which can also be incredibly useful. These are all searchable by Key Stage and also by topic. Your school might already be a member, or in the first instance you can sign up as a friend of Sing Up and gain limited access to their website. To get you started, Sing Up have provided a link to the top ten routine songs1.
Singing is also central to the approaches to music teaching developed by educators such as Kodály and Dalcroze (and there is a resource here from Bethan Habron Jones to demonstrate Dalcroze)2. There are many, many published songbooks, which you may find useful. Some specialist organisations also offer resources. For example, the English Folk Dance and Song Society have a growing resource bank of freely available material linked to English folk traditions and many recorded and printed resources and ideas for developing classroom work at all Key Stages. These resources offer guidance about how to use the material for music education and through the curriculum in mainstream and special schools. The internet is another brilliant resource for finding recordings, karaoke versions and lyrics for millions of songs.