Right now, you cannot set a different goal for individual students within a class. However, you may want to consider this option. If a student or a small group of students are always working at a different level than the rest of the class, you can create a separate class for them to work in on a daily basis.
Create a new class.
Move the students needing adaptations into this new class.
When creating and assigning work, assign the assignment to BOTH the old and new class.
Set an appropriate daily goal for the old class.
Set an appropriate daily goal for the new class as needed (higher or lower than the old class).