Story with children that have magical powers imbued by wearing stones around their neck

I believe I first read this in the mid/late 80s. Here what's I can recall:

• It involved a boy and a girl - I believe 7-10 years old - I don't recall if they were friends or brother and sister


• They were given a "stone" on a string necklace that enabled magic and protection of some sort from some middle-aged/old-aged guy that lived in town


• One plot is where an evil "magician" of some sort comes to their school and turns the assembly into an assortment of snails/other creatures, but their "stones" protect them from the magic


• I vaguely recall that they were both taught transformation "spells" and could turn themselves into snails or something


• At the end of the book, I believe the boy is also able to control the weather - he makes it rain to fill up a water barrel (or maybe he stopped it raining - something like that).

It's a book that my nephew would thoroughly enjoy and would like to get for him, however, the above is pretty much all I can remember these days - I'd recognise it if I see it though...