I also agree that touching up student art work is problematic for all the
reasons already stated but also because it seems that for elementary
students it creates a "No Confidence Monster".
I often have new students who obviously have worked with teachers who
"touched up" or "helped" them by drawing or painting on the student's
paper - these students often try very hard to get me "help" them in the same
way.... After a few classes and a few refusals on my part to "help" on their
paper, that monster gets chased away and the students are back to creating
their own work all be themselves...
Mary in MN