Nice storytelling, enjoyable historical details, a spunky girl character, and a fun (though pretty minor) mystery. I liked how prejudices were addressed, and I thought the ending was sweet. I've read some other Newbery contenders and see this being about equal to them in my mind, in terms of both writing and kid appeal (which is to say, not hugely appealing to kids). 
 There was one detail in a retelling that bothered me a little - that a prune juice mixture was suddenly available for a prank. That didn't make sense to me as it was told, and is the kind of detail that would've bugged me as a kid too.