But you have a limitation on "Edit", don't you ?
You can't edit your post after a max time limit.
And even if people edit the answers, they can't copy the answers because we can compare time.
You can even compare the changes in the posts.
And, why should the examiner be bothered about how many times a student uses eraser ?
Examiner only examines the final answers.
(Moreover erasers can't erase ink : once something is posted, you can't ERASE it, you can only modify it which can be detected)