I signed all my students up and got a class code; however, I cannot get the students on. When they put in the username the site gave them it says that is unavailable

I signed all my students up and got a class code; however, I cannot get the students on.  When they put in the username the site gave them it says that is unavailable.  Well of course it is it is my students.  The problems is he has no password.  I think it is ridiculous that anyone believes a teacher has this much time to play with a website.  Last year this was not a problem and my kids were on in minutes.  This year I am not sure it is worth it.   Can anyone help?   I have tried everything in my arsenal and I am pretty computer savvy.  

WL