lifehack have put together a nice little piece with tips on how to better constructive feedback/criticism.
This would a useful thing to read if you have to take part in any critique of another person's work - as either part of group work or as part of an in-class discussion.
The main points are:
1. Comment on what's right
2. Ask why they've done something
3. Focus on the general
4. Brainstorm fixes
5. Offer an honest opinion
6. Leave it to their judgement
"Critiquing is a skill, just as much as any other aspect of communication. Considering how often we’re asked for our opinions on something, it seems worthwhile to develop the skill to give an opinion without getting everyone in an uproar. While I’d love it if some people would just identify a little less with their work, the truth is that many people take critiques very personally and it takes a deft touch to help them improve a project without everything ending in tears. Whether you’re participating in critique sessions for your company’s next big marketing campaign or you’re headed off to the local writers group, think about how you can give a great critique. How can you really help the person asking for your feedback improve their project?"