[plt-scheme] Commenting code for teaching

Hi,
 
 It may be little bit off topic, but maybe somebody can advise me on this. I teach a little 
 programming and program development.
 The students have some homeworks. I specify a task and they are free to solve it any way
 they want, but ... The issue has been raised that they would prefer it, that I make comments
 on their written code. I am looking for a tool or a way to do this. I could use normal comments,
 but I imagine something more sophisticated, something like a transparent layer on top of
 the code. 
 Does anybody know a tool like this?
 Any idea?
 
 
 Best regards,
 
 Peter Ivanyi