I teach this book at the 7th grade level, because it connects with the history. I do think high school kids could read and enjoy it, but I don't think I would teach it. The hero seems a bit young. I'm not sure of his exact age, but the book doesn't deserve to be relegated to only children's book status. There are some mature themes.
I, too, read this book in elementary school. I love it, and I do think it's a text which should be read by all students. In fact, I just gave it to my niece who is in seventh grade and she's enjoying it; but it's certainly not challenging her to any great degree. I know there are probably high school classes which aren't much above that reading level; however, I'm confident there are other texts which would offer an equally compelling story but have more literary merit (style, theme, etc.) for students at this grade level.
As others have stated, a lot depends on the context of your school, your teaching, your State and your own ideas. Certainly there would be benefit with a specific group of learners that maybe have English as a second language, however, it would be worth inquiring with other English teachers and the administration as to whether this would be acceptable or not. I like the idea of #3 in terms of thinking about including this as part of a multi-disciplinary unit - could work very well, as it would not take too long to do this book and would leave time to do other activities or look at other texts.
I think that much depends on the conditions and circumstances surrounding your desire to teach it. On face value, I think that the book might be a bit lower than the age level specified. Yet, if they are struggling readers or, as previously stated students who are becoming more proficient with English, I think that there might be some relevance in using the text. Additionally, if you wanted to engage in an interdisciplinary unit that encompasses several different subjects, perhaps a relatively easy text would be good for this kind of purpose. I would also suggest that teaching the text might be relevant if it is being dictated from "top down," and if this is so, your decision might be made for you.
To be honest - I remember reading this book in elementary school. I've been teaching high school English for six years and I know for a fact it is not taught in high school in North Carolina, Texas nor Washington State public schools.
Many schools teach it at the 6th grade level because although the subject matter (The American Revolution) is applicable to American History - the reading level is between 5th and 8th grades. I think high school students (even ones who struggle in reading and English) would know this story is a little too easy.
The only way I would recommend it for a high school class would be in a self-contained ESL class - where the students could benefit from the simplified language and story line and also the American history lesson.
If you are looking for something more up to grade level but still dealing with similar historical themes, see the link below. Hope that helps.