Our school works in two shifts, some classes come in the morning, some in the afternoon

If it is already decided which classes come in the morning and which in the afternoon, then you simply need to modify the time-off of the classes:

You may need to raise the number of periods per day in menu Main/School.

If you want the software to determine this which classes the solutions depend on the type of your timetable and has to be advised individually.

Do not forget to check teacher's gaps. The teachers usually need much more gaps per week in this scentario