We have different bells on some days.

It is very common that on some days schools have an abbreviated teaching (usually in Fridays) or simply some periods have different timing (start-end) on some days.

In menu Main/School you can find a dialog for changing the bell times.

If you have different bells on some day, you need to select the period and click the checkbox, then click Edit:

A dialog appears where you can input the times for each day. The default time is displayed at a top, if you do not fill in, then the default time for this period is used.

So fill in just the times that are different comparing to other days.