In Reporting > Reports > Students (tab) > Class Numbers you may find a discrepancy in the number of students in a year group, compared to the total in the forms in that year (see example below).
This would indicate there is a new and/or future student that has been manually added to the system.
Go to Admin > Students > Admissions to see if a new or future student in in this area. If a student is there, check their Admissions Status is set to Enrolled so that they pull through to reports (click in the Admissions Status cell and select Enrolled from the dropdown).
Next, go to the student's profile and assign them to a Form on the Extended Tab, under Core Information. Click Save at the base of the Extended Tab.
When you run the Class Numbers report again, you will find that the total number of students in the year is equal to the sum of the total number of students in each form.