Complex sentence: Although this greatly opened up the choice of schools for students, it had the undesirable consequence in which well-off high decile schools experienced large roll growths, while poorer low decile schools experienced large roll declines.
Split into simpler sentences: Although this greatly opened up the choice of schools for students, it had undesirable consequences. Popular high-decile schools experienced large roll growths, while less popular low-decile school experienced roll declines.