If a student has not yet started a placement test, you can opt to have them not complete one at all. Here's how:

1. After signing into your teacher account, click the "Settings" link in the left sidebar.
2. Select the "Individual Student Settings" tab at the top of the page.
3. Click the box to the left of the student(s) name that you'd like to update settings for.
4. Select the "Update Settings" button. 
5. Click the "Modules" tab at the top of the menu.
6. Click the subject name on the left sidebar.
7. Select the "Start students with no placement test" option next to the "Start student with placement test?" question under the "Initial Placement Test" section.
8. Click the "Save and Close" button at the bottom of the page.

If a student has started a placement test, follow the steps here to delete it.