Login with Google

I have connected my Google Classroom to Khan Academy and about half of my students are able to log in with their Google credentials while the other half is getting an error message that says they already have an account. Their passwords don't fix the issue. They also don't have access to outside email contact, so selecting "forgot password" is not an option. When I tried to manually change their password to their school system password, I got an error message that the password was too weak.