"Block" refers to the time of day - 1st Block is always first thing in the morning.

"Period" refers to the class you have and it is assigned a color.  For example, if (period 1) red is your science class,  the red period will always be science, but it will rotate, some days red will be in the morning, some days it meets in the afternoon.  Grade 6 takes 4 core classes, so they meet every day.  Grades 7 & 8 take 5 core classes, so they meet 4 times a week.

"Flex" is when Band, Chorus, or Orchestra meet.  If you are not enrolled in one of these classes, you are a assigned to a Study Hall for the "Flex" period.