As with any historical fiction, some portions of the book are dramatized or all together fake. However, I find the book to be fairly accurate. It also does a great job of making this period in history interesting. I find that students are willing to learn more about the period when they have a connection to a character that they care about. The book gives insight into the mind of a young man during this time period and allows the reader to absorb the feelings and events of the time period through his eyes. I think students are always more interested when they have a reason to care about history rather than a dry presentation of facts.

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:

http://books.google.com/books?id=3kZj1uiCeGEC&pg=PA79&dq=johnny+tremain+accuracy&hl=en&sa=X&ei=1lorT4isHojm2QXU66yHDw&ved=0CEUQ6AEwAzgK#v=onepage&q=johnny%20tremain%20accuracy&f=false

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.