I've been to some of the international school open days. ISF, CIS are intensive Mandarin, Canadian do about 45mins per day and they tell me that the level of Mandarin is quite good. Harbour School told me they do 45mins a day too (4days a week), so they are not that bad,
Compared to other Int'l schools they do teach a good amount of Mandarin. ESF, which teaches Mandarin as a far-off second language, Australian just kind of brushed me off when I asked about the level of Mandarin, so I presume it's not good.
But I hear there are lot of Chinese students in ISF and CIS. Harbour has more Western students (the majority), which I also considered for my child as their peers will influence my child's cultural upbringing, so that is something to consider too.