If your account has permission to reset student passwords start by going to https://admin.google.com
Login with your school email account and password.
Once logged in you can either click on users to start browsing for the student, or just start typing their name up in the search bar:
Once on the individual student's account page click the reset password link on the left side, then click the slider to not automatically create the password, and finally set a new password for the student to use to logon. They will be required to change this password after logging into their device.