Problem with breaking time and the lessons starting after that

My timetable is like this:

-----working hours-----
8:45 - 9:30 (first lesson)
9:30 - 10:15 (second lesson)
-----breaking hours-------
10:15 - 12:00 (coffee break)
-----working hours----
12:00 - 12:45 (third lesson)
12:45 - 13:30 (...)
13:30 - 14:15 (...)
-----breaking hours----
14:15 - 15:15
-------working hours-----
15:15 - 16:00 (...)
16:00 - 16:45 (...)
------breaking hours-----
16:45 - 18:30
---- working hours----
18:30 - 19:15 (...)
19:15 - 20:00 (last lesson)

That are the lessons that people should be able to book.
Time base: 15 minutes
Service: Lesson 45 minutes

In the working time tab i configure all the time like my timetable... but there's a problem
When the second lesson finish in the calendar it's ok but when the third lesson start after the break it start at 11:45 and not at 12:00.
The same problem with other break after the second break the lesson in the calendar start at 15:30 and not at 15:15.

I tried several times please help.

Jacopo.