Hi Nellie,
I think this is a great idea!
Regarding tutor/lecturer comments, there are a few alternatives you could think about;
1. It would be possible to design a "Comment template" which could be inserted on the actual page -- something similar to the note template (
http://wikieducator.org/Template:Note ). In this way you could easily use a different colour for comments. The downside is that the comments may disrupt the flow of the essay but have the advantage that they can be deleted by the user once they have attended to the comment.
2. You could use the <ref></ref> tag to insert comments with the <references/> tag at the bottom of the page to list all the comments. Student's may become confused with the <ref> tags inserted in the text -- although if explained in advance, this shouldn't be a problem.
3. My personal preference would be to use the talk page -- this way there will be a permanent record of the comments. The downside is that comments will not be inserted in the text of the essay.
4. A combination of the above.
Students deleting text shouldn't be a problem - its easy enough to roll back edits. As lecturer you will have a permanent history of all the students work, including inadvertent deletes <smile>.
Regarding email notifications -- students would need to add their essay pages to their watch list and check the email notification box in their user preferences. You could easily compile a step-by-step instruction sheet or tutorial on how to do this for the students. Students could also sign up for one of the online L4C workshops for basic skills training.
Can't wait to see the project in action.
Cheers
Wayne