Futuregirl – unless things have changed over the years, MMY used to be either the most or one of the top two in terms of academics. I’m not trying to scare you away – it’s a great school and it would be great if you went – but it is super-academic, so you have to be prepared for that.
In terms of whatever it is your sister heard, keep in mind that most schools have different types of girls in them. It would be very hard to find a school that hasn’t had any “not-good girls”. I haven’t been involved with MMY in several years, but they used to have great girls there.
In terms of your wanting academic but not overly so, it seems to me that TD or MT would be more matim. Shaalvim might also be a possibility, but it’s possible that it’s more academic than you are looking for (although it is less than MMY)