I am about to start teaching this book again. I am a literature teacher first, and a history buff second. How accurate is this book?
The small details, such as of daily life, are accurate to the time period as well, which provides a richer depth of historical accuracy. Things like apprenticeships and food and workmen (e.g., weavers, bookbinders, clockmakers etc) are details that add richness and depth to historical correctness and context.
Ester Forbes's Johny Tremain is certainly full of historical accuracy. Not only did Johny Tremain win the Newbery Medal in 1944, Forbes's biography of Paul Revere won the Pulitzer Prize for History in 1942, and a great deal of research on Paul Revere was later used in Johny Tremain.
I think that the majority of the historical detail seems to be very accurate from my point of view. There are no major inconsistencies that I am aware of, apart from the fact that this is a fictionalised account and therefore has been changed as a result to fit this genre.
I did a search of the web (both Google and Google Books) and turned up one reference after another to the historical accuracy of this novel. In one case, for instance, the Massachusetts Historical Society vouched for the accuracy of the book's presentation of John Adam. Here's a link to an article that directly addresses this issue:
Of course, much of it is fictionalized for dramatic purposes. But I don't have any quibbles with anything major in the book. James Otis really was a bit crazy by that point. Forbes doesn't go with the Longfellow version of Paul Revere's ride and she at least implies that there will be one other rider other than just Revere. I think that it sort of overstates the amount of Indian dress that was worn, but there is debate on that issue. But the general ideas of the book are sound so far as I can remember.
Honestly, I remember it mostly from the film, so I am two levels removed from pure history... I imagine, thinking back, that the book is dramatized for effect, and also sanitized since it was meant for children. Probably your best bet is to teach it as a historical drama, and teach facts alongside the book. Depending on how much time you have before you start teaching it, you might read through it yourself and annotate the historical aspects based on your own knowledge and research; this will give you a base to start discussion topics and answer questions about the history.