My DD1 did the IB at St Leonards and DD2 is in her final year doing it right now. DD2 was in year 6 when she moved to St Leonards and she loves it.
The IB is not an easy choice. The pupils have to choose 6 subjects: 3 at higher level and three at standard level. They must do maths, english, a science and a language (DD does biology and Spanish) she also chose history and economics. On top of this the do 4000 word extended essay and a Theory of Knowledge essay plus they must do extra curricular stuff for CAS (creativity, action, service).
It is very hard work but can pay off if the student is a good all rounder. DD1 went to Glasgow after her IB and achieved a First. She said her first 2 years were a doddle as she was so good at self directed study, writing essays etc.
I would have to say that quite a number of DD2's cohort moved to the state school in St Andrews to do Highers after GCSE's for a number of reasons. Some didn't want to do science or a language, some felt it would improve their uni application chances, some just would not have managed it.
Beaconhurst is a really small school on the edge pf Stirling uni. I don;t know much about it but a friends DD went there. She was more keen on drama and creative stuff and it seemed to suit her.
DD's best friend from childhood went to Morrisons and loved it. She has just left and is studying dentistry now. I think Strathallan is a very sporty school as is Kilgraston.