The student can no longer access the classroom. They cannot see earlier posts, even if they made them.
Note that previous year classrooms are automatically displayed in the previous academic year’s folder, and content cannot be posted to the classroom belonging to the non-current school year. Therefore, you don’t need to remove students from the classroom to prevent posting. If you don’t want students, parents and teachers to have access to previous years’ classrooms, you can hide them or delete them.