We have a part-time nanny who attends school (not for a Masters) and she is amazing. I found her right here on DCUM so, OP, if you're willing to put in the time and effort to interview, it doesn't have to be a crazy or expensive search. Not having to do daycare drop off, and having reliable care for all the many days my older child's school is closed, has been a major life-changer. We started off paying her $17/hour for 2 kids, 25-30 hours per week, which for us was less than what we would have paid for daycare at that point. I now suspect daycare would be cheaper (after 2 years of raises, etc) but I love our situation and would not go back.
I think it depends largely on, 1, what you're willing to pay, and, 2, what you define as a "high-end" nanny. Does this mean she invests a lot of time in teaching, or in outings, or does a lot around the house for you? Our nanny is fantastic, but she isn't busting out flash cards to do with our kids or teaching them Swahili--she's reliable and consistent, caring, has impeccable judgement, etc--but I don't know if I would call her "high-end."
The caveat I can offer about making this arrangement work is, find someone who's already been doing it. Our first nanny seemed like a great fit, interviewed really well, and was starting grad school for a Masters in Education with all evening classes. It was a disaster--she was a total flake, called in sick repeatedly, showed up late, and quit after less than 2 months. I honestly think she was not prepared for the reality of grad school and could not deal with the workload of working 25 hours a week and a full course load. Our current nanny had already been in an identical arrangement--attended school 2 days a week, worked for another family 3 days a week--so she knew what it would be like.
As for the PP who mentioned a sort of "nanny share," we thought about this, too--but it was pointed out to me that there are a lot of problems with that scenario--i.e. the nanny could easily wind up working more than 40 hours/week with no overtime, and/or there might be weeks each family needs a different schedule--it would be really hard to sort through all of that, I think, so we felt like it was best to go with someone who specifically wanted PT.