This task can be performed only by admin.
Create new classes for the new academic year (say, for example if you want to promote students from 2020 to 2021). The new classes should include all type of classes just like your current classes which may include type of class Main, Bus routes, Extra curricular, Sport, Clubs, Activity ..etc
Login as admin, go to navigation bar, Select manage groups.
Select the class from which the students has to be promoted to next class
Click on the Promote Students Button by going to Actions like in the below screenshot
You can either select all the Students or Select Student/Students manually who needs to be promoted to next class
Give the start date for the class for the next academic year.
In promote to class, Select the higher class to which the students are to be promoted(May be from class 8 to class 9)
And click on Add button..
Manual selection of students to promote to new class