Hi, I would encourage you to have a look at the American International School (). My children have been visiting there for 3 and 4 years, respectively and our overall experience was okay. They have small classes (in my kids case 12 and 13 students) and more often than not teachers are well motivated. School language is obviously English and they offer French and Spanish (unlike most other schools where you will be forced to have Afrikaans or Zulu, which most expat kids won't really have much benefit of). Their school year starts in August and not in January (also unlike almost any other school), which makes it easier for kids from Europe to settle in. In high school they offer quite a broad range of subjects and they have a full-time guidance counsellor who helps students choose the right range of courses for their respective choice of College disciplines and who also helps with University applications. I am not aware of bullying (and my daughter might well be a target given that she has some behavioural issues) and they are quite accessible when you have a problem or need to make special arrangements (in our case spending a whole term abroad each year).
Downside is a huge amount of homework in middle school (although my kids say it got better in High School), and that certain disciplines are missing (e. g. economics is a weak spot and sex education seems to be a bit like talking of flowers and bees). They are also quite expensive as compared to other schools (but I think this comes with the small classes) and in that regard they might take a tough stand when you have a disagreement (I had to take a lawyer to get a refund I was entitled to, but I don't feel this dispute backfired on me or my kids).
Another factor is that they are in a less densely pouplated area so that traffic in the morning is not too bad (if you choose where you stay wisely).
Under the bottom line my experience has been okay.
There is also the International School of Cape Town in Wynberg which is moreless working according to the British System, which has bigger classes (23 - 25) and is less expensive.
If the cost is an important factor, you might also consider the German International School. They do offer classes for non-German speakers and are cheaper than other International schools as they get subsidies from the German government. But in my humble view (and I spoke to them when I came to Cape Town and I know families whose kids were there) they can not compete with the 2 schools I mentioned before (but of course they still offer decent academic standards). Also, Afrikaans is mandatory. you can also forget about special arrangements like e. g. missing out on their tuition for part of the year.
Parklands College in Table View might also be an option, but they also have mandatory Afrikaans tuition and they start in January.
Good luck to you and your kids
Peter