An acquaintance of mine is trying to track down a book they read in a library around 1980 or so.
It is a young-adult story following a set of kids (three siblings and some friends) travelling on a school bus and having increasingly magical adventures. At the end of the story they meet a man who holds some kind of gem or stone in his hand and they end up back near home.
The story may have been set in Australia during a wildfire, in which case the adventure starts with the wildfire cutting the kids off from home so they have to take the long way back. However, it is possible that my acquaintance is conflating two stories, one about a wildfire (which might not have been SFF, possibly Ash Road) and the one about the magical adventures.
Some specific details:
The school bus driver / teacher had different names based on the kids smearing the name, e.g., something like ‘Mr Smith’ into ‘Mrs Mit’.
The three siblings went to a candy shop and bought something from high up on the shelf, possibly lemon sticks, and ended up getting the shopkeeper annoyed at them for sending him up the ladder again and again.
One of the kids had a pony that died in the fire, but they didn’t find that out until the end.