Both groups have to finish the education at the same time!

By default the software can generate a timetable where one half has English on Monday 6th period, while the other half goes home after the 5th period. The next day the groups are switched:

This can greatly increase the chances of generating a timetable.

However if some teacher has to go to lunch with children then this behaviour is probably not acceptable for you, because you will need two teachers for escorting children to lunch each day(one for each group).

In this case you can forbid this behaviour by checking that 'Groups must finish the education at the same time':