Hi wanderer123,
It would be easier if you just named the schools — everyone else does — but the principle of the following advice remains the same. I see your dilemma but you seem to have answered your own question. There’s a chance you could forego a 'better' school by sticking to your word but it’s not as though what you’ve been offered is bad:
You’ve already had some rejections and there’s no certainty you’ll get any more offers. As you pointed out:
Besides, its not as though the implications are unclear:
The only danger is that the offer from School B doesn't materialize. With this in mind, since you accepted the School C interview before the School B offer, it would seem ethical to go ahead with the School C interview. If the School B offer subsequently fails to appear (and you've actually checked with them about this), and you're made an offer from School C, then you can accept School C. However, if School C makes you an offer and School B's offer has materialized then you'll turn down School C with the moral victory of knowing that you could've gone to a better school (C) but opted instead for a good school that believed in you from the start (B).
Hope this helps,
MC