The samples for critique should not come from your own students. You should generate these samples or obtain them from another class. Be sure that whomever wrote the sample has given you permission to use it in class, and cross off their name before making copies.
It's useful to do critiques at the end of a unit, or just before a workshop. Have students read the entire sample piece of writing before coming to class and ask them to comment on how well it meets the criteria for the assignment. Consider various approaches to critiquing the sample and choose the approach that works best for you and your students:Techniques for teaching students to critique