You may notice that while students are working on Freckle, their coin totals drop to zero. No need to worry - your students didn't actually lose these coins (unless they spent them in the store). This happens because the students are working on a new student session.
At the end of each student session (level completed, trip to store, new set of questions, etc.), the coins earned by the student during that session are added to that student's daily total. This daily total can be seen on the teacher dashboard (Sessions tab) and on the student dashboard.
These coins are also included as part of the cumulative total, which can be found on the teacher dashboard (Report Card tab) and the K-8 student program (in the Piggy Store). At the end of each day, any remaining coins from that day's work remain part of the student's cumulative total and can be saved or spent.