In my developomental and basic English classes, I have arrived at a check
list which indicates the seriousness of errors: failure to show
pre-writing and a plan results in a No Grade: I do not read such
papers; loss of unity, lack of organization, inadequate development,
primer sentences--these are all automatic F's; fragments, comma splices,
fused sentences, subject-verb disagreement, and verb form errors will
probably result in a D; pronoun disagreement, apostrophe problems, common
comma errors, spelling, and glossary of usuage errors will usually hold a
paper to a C.
 
The system works pretty well.
 
Occasionally, I will return  papers with the number and type of errors
recorded at the top of the page but not marked--with the offer of an
improved grade if the student finds and fixes the errors.  The offer is
seldom accepted--and when it is, the acceptor is generally already a
fairly good student.
 
Red ink?  Yes, often.  But when I see a paper more than once, I change ink
color; the variety of colors makes a statement on it own.
 
Good luck with your project.