Users at same level can edit each other's posts & comments

I am a teacher. Each student in my class is an ‘Author’ on the class blog.

By default, apparently, all Authors are able to edit any comment or post written by any other Author.

This is far from ideal, obviously. I would like Authors to be able to edit only their OWN posts and comments. I have found a couple of very old threads about this issue, but nothing recent and no solution.

Theme: Atahualpa. But this issue does not seem to be related to any particular theme.

Plug-ins: Akismet, Category reminder, Featuring CountComments, Private Messages for WordPress, WP Authors, WP Likes, WP Quicktags. But again, I don’t think the problem is related to a plug-in; it seems to be an issue with WordPress itself.

Any help with this would be much appreciated.

Eric