Yup - a "block" should be the same thing throughout.  A schedule (a collection or blocks) represents all teacher/student time commitments over the entire week.  A calendar (a collection of "slots" is how I'm visually representing this schedule to the user - it shows all of the time commitments for the currently-selected student group.  Put another way, a block contains one or more slots for a given student group, for a given teacher (unless we're talking about something exceptional like recess).