Complex sentence: Nanyang primary girls are required to keep hair short enough not to touch their collars, unless it is neatly tied and if they have a hair-pass that can be acquired in the form of a letter from their dance or ballet teacher.
Split into simpler sentences: Nanyang primary girls are required to keep hair short enough not to touch their collars. BUT, they can also keep long hair as long as it is neatly tied and if they have a hair-pass that can be acquired by a letter from their dance or ballet teacher.