If your student isn't be adding to their Google Classrooms automatically, it's possible that there is an error with their email on their SIMS page.
If it is missing from the student's SIMS page or if there is a misspelling where it is inputted, this will need to be updated by the MIS team so that the details are correct and then the sync should correctly assign the student their classrooms.
If their classrooms are still missing after this is updated, or if the details were correct already, please let us know.