Be compassionate and helpful to your colleagues. Start with what works, end with what doesn't. There is nearly always always something right with a storysomething that will appeal to you, something that seems apt. That's where you should begin your criticisms. Then work your way around to what's not working.
Nobody learns anything that way.
In writing your fiction or in writing commentary, good grammar, correct spelling and proper punctuation are not optional. Anything less is abusive to the reader. A poor command of the essentials of good writingand grammar, spelling and punctuation are essentialrepresents a breech of that contract. In your exercises, your stories, your critiques of your classmates' work, observe strictly the letter of this law. Don't make vague, general comments about a piececomments you can't support with examples from the text itself.