QUOTE(dorfmouse @ Nov 20 2007, 11:11 AM)

I feel so angry and sorry on your behalf. How could the school management (supposedly composed of experienced educationalists) knowingly create such a situation for you? 20 reception age children, armed with recorders, for one hour at the end of the day, with an inexperienced teacher ... it's mind-blowingly incompetent management and irresponsible to both you and tha children.
Again, you have had some excellent advice.
One of the first things I learned was to be tough at first and show you mean business, then you can relax a bit afterwards - never the other way round! This means it is now extra difficult for you but dorfmouse is right, it's very bad management on the part of the school and the head/music co-ordinator should be supporting you in this.
This sort of scenario is not uncommon with after-school clubs that are not run by one of the school's regular teachers. I remember a French club where the teacher only had about 6-8 children (aged 5-9) and they just ran riot completely.
It may even be that you have reached the point where you tell the head/music co-ord that you cannot do this any more without things changing - either the length of lesson/number of children/support re discipline or all three. Find out what the discipline policy is in the school as children respond well to consistency. The children have to know that there will be consequences for bad behaviour.
I have a very 'lively' Year 2 class at the moment - lots of gorgeous children and a very bright and musical class, but a significant number who are very immature behaviour-wise - so both their class teacher and myself are really cracking down on them hard. I warned them that ANY stepping out of line meant that they go back to their classroom and missing out on our games - and there have now been a few weeks when that has happened and they're getting the message! It has to be - warning then follow through with the promised consequence of the bad behaviour. It is impossible for any real quality learning to take place if the children are behaving badly.
Busy Bee is right (btw, glad your interview lesson went well

!) in that changes of activity can be 'dangerous'. Teaching children to make a circle is an art in itself!
Best of luck and keep us updated.