Go to Settings, and select School Calendar. Now, when blocking a date, you can select single or multiple periods if your school uses more than one Flex period. This way, if you want to block a day only for a certain period, maybe because of a school assembly during that period, for example, it won’t affect other periods not selected. On the other hand, you can still choose to block all periods if you’d like.  And you can remove the blocked dates by period as well, not just the day of the week.