A lot of people keep throwing around the word "sexist" without understanding what actually is and isn't sexist. For example, there was large amounts of rage over the first image of Equestria Girls to appear on the internet and how sexist it was, without giving any actual reason why it was sexist. (Twilight wearing a dress isn't sexist. Twilight being skinny might be, but the show is stylized enough that the issue might be a moot point).
A lot of this popped up again for the EQG trailer, with people being outraged about sexism, when I personally don't see any sexism as of yet. Again, the Mane Six being humans are not sexist, them wearing dresses are not sexist, and them being in a high school is not sexist. Believe it or not, there are many teenage girls that wear dresses who also attend high school. What is sexist, however, would be things like the girls wearing skimpy outfits, heavy focus on romance and girl stuff as if they are the only things that matter in the world, girls being weak and deserve to be "rescued" or be "taken care of" by men and/or society, and the implication that being skinny and/or beautiful is more important than other things. I have seen no evidence that any of this is the case in the trailer alone.
I think part of the problem of EQG is what's in the trailer: Twilight and a bunch of Mane Six lookalikes are teenage girls in a high school, and a boy who is strongly implied to be a possible love interest makes an appearance. This kind of plot has been used in thousands of highly-sexist ways in the past, but people are making the (fallacious) jump that this plot is automatically sexist in itself, which is not actually the case.
TL;DR: In my opinion, there is not nearly enough evidence that Equestria Girls will be a sexist cliche high-school teen girl plot. There's a strong chance that that will be the case, but it will be impossible to confirm until the movie is released.
Addendum: as for Princess Twilight, my argument applies to that as well: we have no idea what her duties as princess will entail, so, while it's still possible that it will be tea parties and makeovers galore, it will be impossible to know whether she will fall into that cliche until Season 4 airs (or is leaked), and to automatically write it off otherwise is likewise close-minded.