As a teacher you have the ability to manage your student's passwords from the roster tab in the teacher dashboard. Follow the steps here to see how it's done. Start by heading to the roster tab in the teacher dashboard for the class that you want to update student passwords in.
Use the checkboxes to select the students that you want to change passwords for (you can use the checkbox at the top left corner of the roster to select all the students in the class).
Click on the Choose Bulk Action dropdown in the top left corner or the roster tab, select change passwords, and click the apply button.
You should now see an overlay with a couple selection settings for the change. The default is to update each selected student account with a different auto-generated password. The alternative is to update all selected student accounts with the same password that you specify.
Make a selection and then click the update button.
You should now see the password field for each affected student in the class turn blue with the pending changes. Click the save button to finalize the changed passwords.
You can verify that the passwords have been updated by double clicking on the password fields for the students.