Students have been made teacher in your course or student role for site has been granted permissions to view them.
Check permissions on student role - every where ... course level as well as site wide and maybe overrides you did for students.
/admin/roles/manage.php
/admin/roles/assign.php?contextid=1
Check a known student account (or one that you think is student)
/admin/roles/check.php?contextid=1
'SoS', Ken