Teacher Discussion Forums

I am dealing more and more with children of all ages and I would like to cater the classes a bit more, narrow down the levels etc. But I have so many kids of all different ages and levels. Does anyone know of any children´s level assessment tests out there for ages say anywhere from 6-15?
Or what´s the best way to assess a child´s level? I have tried to surf the internet for clues, but there doesn´t seem to be much helpful information. I have consulted Oxfords books on teaching young learners.

Thanks in advance for any help you can provide,

Sarah