Three local schools have asked me to come and speak to their students about creative writing recently, between the ages of 10-15. I turned all three down because of my absolute hatred of public speaking. I can only terrify myself once a year with a public appearance of sorts, saving that up for late in 2013
Back to topic... The schools that approached me are relatively close, all three are within 30 km, and the hint of payment never came to the surface, nor did I even think about it until reading this thread.
I don't know how local your schools are Rob, but I'd mention your fee before booking anything concrete.
As for what you should charge, it can vary. On one hand your trying to make a living from your writing, on the other this is a form of advertising.
It is not going to make you rich, but if I had to charge I'd go with;
20 cent(euro) per mile traveled.
20 Euro per hour (If doing multiple classes I'd go with 15/hr as you'll be repeating the same thing)
15 euro - Cost of a decent lunch
Bring a memory key with what I want the school to print off for me, no cost on materials.