Complex sentence: Students obtaining a satisfactory grade will be promoted to Form Six, where they prepared for the Hong Kong Advanced Level Examination (HKALE) (colloquially "the A-levels"), which was taken after Form Seven.
Split into simpler sentences: Students obtaining a satisfactory grade will be promoted to Form Six. They then prepare for the Hong Kong Advanced Level Examination (HKALE) (colloquially "the A-levels"), which is to be taken after Form Seven.