In order to assign fee type to grade, please follow the steps mentioned below,:
Go to Fee Management > Fee Types
Click on three dotted icon
Click on "Assign Fee Type to Grade(s) or Student(s)".
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Click on Grades.
Search for Grade name and select the required record.
Click on Assign button.
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Fee is assigned to the selected Grade. (click on upward arrow)
You can also refer to article How to assign fee type to student for details.
Note: When a fee type is assigned to a grade, then for all the enrolled students in the grade, if any student has an open & unpaid invoice, then assigned fee type is added to the invoice. Otherwise, a new invoice will be generated.