How do I control when my students can review their assignments?

You can control when students review their assignments when you create the assignment.

Big Ideas MathTopicInstructor Help
Big Ideas Math Instructor Help

In Step 1, in the Release for Review section, you can choose On Submission or By Teacher. If you want students to automatically review their assignments, leave the default selection of On Submission, and students will be able to review their assignment as soon as they submit it. If you want to manually release the assignment for review, select By Teacher. Once you are ready to release the assignment for review, click Release Review for that assignment on the Reports page.