From your Google Email click the settings icon and select Manage this Domain
You may be asked to enter your password. Once logged into the Admin site click the Users tile
You will then see a list of your students. To change the password of a student move your mouse over he student and click "Reset Password"
Uncheck "Automatically generate a password" and you can set a temporary password for the student. Ensure that "Ask for a password change at the next sign-in" is checked so that students will have to use a unique password.