If a kid is weak on a particular subject, a private tutor (whether group or individual) could help to clarify his concept and help lay down a foundation for his future study.
For SAT, Tofle etc, the tests demand the knowledge which you could have from your different course- languages, humanity, science and maths subjects. The engagemen of a tutor to train a kid for such tests appears to me that he is doing the things the other way round. Such training may not help them in building up the foundation.
As far as I know and from my observation, the background of CIS and ISF students are very similar - a small population from wealthy families in town, some from celebrity families and the rest (more than 70%) are from upper middle class. This is not surprise as the tuition fees of these 2 schools are very close too.