there are the free colourful postcards,available from Trading, that are often better and getting girls attention than a plain letter. Stick your own details on the back so they can contact you before going straight to Join Us
I did a school assembly this morning and it was actually really good fun. The kids were enthusiastic and asked loads of questions. Mine was a KS2 assembly so slightly older than your crowd would be, but I'd definitely recommend it as a way to introduce children to Guiding.
I do work myself so know it's not always possible but if you have anyone in your District who can do this then it is well worth it. Perhaps, see if you can attend PTA or similar meetings that take place outside normal school hours?