I'd always say with a child over the age of 12, their independence in getting to and from school / activities should be a major consideration, so living on an easy public transport route to school should be a consideration. That would put St John's in 3rd place, unless you live virtually next to the school, but that would mean your commute to work would be a slow cross country drive.
ISB living in Watermael-Boitsfort would be a good choice, lots of good public transport passing school, easy access to E411 for work. Warning, if joining the swim team, means early morning drives to Waterloo!!! Tennis team will also be expensive. After school activities are almost all fee paying at ISB.
BSB and living in Tervuren or WSP/Wezembeek/Kraainem would also be a good choice, public transport by 44 tram or bus from these areas or cycling to school. Warning, swim team currently means afternoon at Evere pool once a week, many of the best school swimmers join the local francophone club in WSP called CNSW, French not required, so many anglophones and other languages there. If you're here longer than 2 years, BSB will be the only international school in Belgium with its own 25m competition pool and the best sports facilities. For next 2 years, it will be dust and mud whilst these facilities are being built. Tennis is very low cost at BSB as the school team is subsidised by the school. After school activities are often free at BSB.
Spanish would be best supported at BSB.
Horse riding, not a problem, lots near all schools.
I'd look at ISB or BSB and the areas for living. Both are going to be E411 commute, unless you want to take the bus from WSP which runs regularly Mon-Fri.