Bettyboop - The act of praying and the use of the term 'Let us pray', whether with hands together or not, heads bowed or not, is an act of worship. The use of the Lords prayer, and of 'Amen' is an act of worship. Hyms, harvest festivals, christmas services, carols etc, they all pre-suppose the existance of a deity at all. They are scripturally defined terms and rooted in religious worship. They do not, as you state, leave it open as to whether one believes at all or not.
Would any of you be happy if an Imman came to lead prayers to Allah, or a Sikh leader leading worship to Vāhigurū, or a Catholic priest, or a Rabbi etc and all children were obliged to take part? Not just to witness a service, but to take part? Because that's exactly what happens when teachers, heads etc are obliged to produce assemblies using any of these means (prayers, hymns etc). And just because something is not 'very religious' does not mean it is non-religious.
MrsZ - I've read every single post on the thread, and whilst I agree with you that most schools try to avoid any overtly religious content (and quite rightly imo), it's the wider issue that I am really angry about. The fact that schools are hidebound by a ridiculous law which means they have to have some kind of group 'worship' at all, that they then have jump through hoops and to try not to offend. The entire clause needs to be scrapped. Group contemplation on ethics, morals, etc is great and we probably need more of it, but we do not need to have that cast under the umbrella of religion.