Best IB schools for oxbridge/IVY leage schools Watch
Some schools do have a track record for getting very good results in IB, however UK universities at least will consider grades in context; doing well in national terms but averagely in terms of the very high results of a particular school is technically not as good as doing well in an under-performing school. In any case, the school I'm most aware of in the UK for getting good IB results is Sevenoaks School. As above though, doing average compared to your peers at Sevenoaks, even if it's very good overall, may not be viewed as favourably as doing much better than your peers at a state comprehensive and meeting the admissions requirements for Oxbridge.
Again, for Ivy League this doesn't necessarily apply as they generally want to see you've held notable leadership positions in long term extracurricular commitments and have distinguished yourself within these commitments beyond merely turning up or being elected to a leadership position. Ivy league admissions are very different to Oxbridge (or even UK generally) admissions, which tend to focus much more on purely academics and for Oxbridge, only on extracurricular activities specifically relevant to the course you're applying to.