Well I'll be.
You need to login with htmlclassroom.t35.com as stated in the welcome email, several other pages on the site, and by TheDarkness a few hours ago.
I misunderstood and thought you meant to go to
ftp://htmlclassroom.t35.com , but I just went to
ftp://t35.com and used htmlclassroom.t35.com for the username (even though the welcome email says my username is simply "htmlclassroom" and it worked.
So..... I decided to go to t35.com and try it there, and that worked as well.
I think they need to change the welcome email to say password: blahblah
.t35.com instead of just blahblah.
I'm usually not this dense, I assure you. Thank you all for all your help, and your patience!