This error arises when no students (either Pupils in Class or Guest Pupils) have been recorded against a class in the Class Information area of the Census. Go to the Spring Census front page, and click on Class Details.
Check against the class name mentioned in the Error (in this example, Banana Class). Look in the columns for Pupils in Class and Guest Pupil Number. There cannot be a zero recorded for each category, as there must be some students in the class. Click on the pencil icon in the Actions column to edit the entry.
Once the data has been amended, save by clicking on the tick in the Actions column.
Recalculate the Spring Census.