I think it's reasonable to rate it R, because the rule is one f-word = R. Some parents, for example, might feel very strongly about a 14-year-old hearing that kind of language even briefly, so it's necessary to call it R rather than misleadingly label it something tamer, like PG-13. However, I wish there were a way of making it clear to people that when they say it's "rated R for language," that means one very brief scene, not that the whole movie is filled with profanity. There's definitely a difference - I wouldn't be enthusiastic about watching a movie with the f-word appearing every 10 minutes, but I was fine with its brief use here.
The R rating aside, I don't think it's appropriate for schoolchildren (assuming you mean actual children, not high schoolers). Even if the language were ignored and they rated it G, I wouldn't want my 10-year-old hearing the f-word, just because I wouldn't want him or her exposed to it. IMO, the solution to letting children see it (and I do think it would be a worthwhile movie for them on several levels) would have been to leave the profanity out, or produce an edited version, not just change the rating.
I saw the movie two nights ago and LOVED it - seeing it again this weekend! I thought it was very well done, and I was very impressed with the attention to minor historical details. For example, Elizabeth and Margaret brushing their horses before bed - historically a nightly ritual for the girls.