https://support.khanacademy.org/hc/en-us/sections/4404526936333-Classes-and-Assignments try this place if it's helpful.

Hi Nancy,

Are you still experiencing this issue? If so, could you please submit a support ticket so that our support team can take a closer look?

Please be sure to include the username / email associated with the student account and the name of at least one assignment that they are not able to access.

Thanks!