Double-check the spelling of your email address, and make sure CAPS
lock is off.
Try logging in with your school email address. Most of our users are
signed up with their school email address rather than a personal email
account.
If your school or district uses Google Apps for Education, you'll be
able to use single sign-in. Hit the 'Google +' button to log in with
those credentials. (Note: if your school email inbox looks like GMail,
you're using Google Apps!)
If you're still having trouble, just click on the Live Chat button up
top.