I really don't think that is the reason. Where I grew up 99% schools had two shifts, and sometimes even three shifts. Boys and girls were not separated, although my country is one of those where >80% peple are christian.

Our school day was up to six hours long (7 school hours (45 min each) + 5 min breaks in between). We received generally the same quality education as many other Central/Eastern European countries. Btw., the reason schools introduce extra shifts is to be able to accomodate more pupils in same classrooms.

Merry Christmas!