Complex sentence: When MCS and SCS merged in 2013, the former MCS schools kept this uniform policy while the existing SCS schools did not, since the suburbs plan to form their own districts and leave SCS within a year.
Split into simpler sentences: When MCS and SCS merged in 2013, the former MCS schools kept this uniform policy while the existing SCS schools did not adopt it. This is due to the fact that the suburbs are expected to form their own districts and leave SCS within a year.