To provide feedback on your classmate's article in their Sandbox, you'll use the Talk page in their Sandbox.
1) Log into your Wikipedia account. This will make it easier to sign your feedback.
2) Follow the link your classmate provides to their Sandbox. The link should look similar to this one:
3) Review their article, then click over to the Talk page, using the tab attached to the top of the page.
4) Compose your feedback. Add ~~~~ at the end of your entry to sign and date your feedback.
5) Click "Save" to submit your feedback.