Students will go to international schools after K2. For ESF, most of them go to Bradbury School or Quarry Bay School (depends on the catchment zone). The rest of them will go to CDNIS, GSIS etc. I'm quite confident that the school prepared children well for international primary schools interviews, students' English fluency should be good enough and could be able to pass the interviews. The percentage of foreign (expat) children are quite high in fact, communicating in English between students shouldn't be a big problem. Even local children could pick up very soon, one major concern for local parents is the Chinese language learning, since St. Teresa doesn't provide formal Chinese (Mandarin) lessons, some parents may consider teaching by themselves, join some outside school Chinese lessons or even attend another local kindergarten in the afternoon after the morning session at St. Teresa (or vice versa, attending a local kindergarten in the morning and attdend international kindergarten in the afternoon). But of course, attending two schools each day is too tough for a little kid and it's not recommended in my opinion.
Seldom of them go to local primary schools since St. Teresa is an English kindergarten adopting non-local curriculum, moreover, the school doesn't operate K3, therefore if a child has to attend a local primary school, s/he must transfer to another kindergarten for K3 before going to local primary 1.