To see how many students are present per form on a particular day, go to:
Reporting>Reports>Attendance tab>Custom Attendance Report
You will then be able to set the date range for a particular day, if you want to include just AM/PM sessions or both.
In the "Group Students by" column, select "Form Group"
Click the "Choose" button at the bottom of the screen to run the report - this will provide a table with numbers of students present and those absent for each form group on that day.