--- /usr/lib/python3/dist-packages/ucsschool/importer/mass_import/sisopi_user_import.py 2023-07-20 15:10:35.724385090 +0200
+++ sisopi_user_import.py       2023-07-20 15:10:25.024844749 +0200
@@ -150,6 +150,9 @@
                 imported_user.schools = old_user.schools
                 if self.config["school"] not in old_user.schools:
                     imported_user.schools.append(self.config["school"])
+                new_classes = copy.deepcopy(old_user.school_classes)
+                new_classes.update(imported_user.school_classes)
+                imported_user.school_classes = new_classes

         return super(SingleSourcePartialUserImport, self).prepare_imported_user(imported_user, old_user)