I just helped my friend's kid to apply, the degree of difficulty way beyond our expectations. But you are right, more openings for senior grades I was told during the application process. But the adjustment process is also longer for such grades, may be that's the reason for more openings.
其實會不會考慮St Margaret, 啟思etc, 比較容易快速適應。因為他這個年紀轉去IS, 很快又要面對高考,還要顧及適應問題,我見不少由local school (even名校神校)剛轉去IS的,頭一兩年也要補下英文,而且初初也是local school 和local school 自己玩,都要一些時間融合。