Good afternoon ma'am, this place might be helpful. https://support.khanacademy.org/hc/en-us/categories/200175820-Teachers-Coaches

Hi Mrs. Bieler,

If you removed students from an existing assignment (unchecked their name and then clicked Save while editing an assignment), then I'm afraid this permanently deletes their assignment progress.

When new students join your class, they should automatically be given access to all active assignments in the class (those that are not past the due date). To add a new student to a past-due assignment, I suggest creating a copy instead in order to avoid accidental score loss when editing the assignment.

For more information on editing and managing assignments, please see this article: https://support.khanacademy.org/hc/en-us/articles/360031121631-How-do-I-edit-copy-and-delete-assignments-

Sorry I couldn't be of more help! Please let me know if there's anything else I can do for you!