Northwestern is a very good university, ranked usually as in the top 10 in the US.
As I said before, it is statistically much harder to get into than a Russell Group university on average.
You need very high SAT scores and good grades in your subjects, unless you on a sports or music etc scholarship, which doesn't apply to MM. Northwestern's theatre programme, which MM was part of, is especially highly regarded.
That is evidence that she is pretty academic/did well academically, and it's and can't be rationally discounted. Harry's GCSE and A level results as relied on as proof to say he didn't seem particularly academic.
I have a feeling that her children, or at least one of them (either one), will do well academically. Personally I don't feel it's that hard to do at school-age when you work very hard, have good teaching, the right environment, lots of tuition, and are taught how to point-score well in certain exams.