Do some websites block school building/classroom IP networks because they mistake us for possible DDOS?
Yes.  I have a web development class.  If more than 5 people connect to the free hosting site, it stops anyone else from connecting via FTP, since we appear to all have the same IP address.  Multiple connections from the same site can be a script running, and most users will not appear to have multiple connections.
I do: Demo using the FTP client, go through some of the issues with connecting (error messages etc.) and then, because of this restriction, I will either:

1) Manage the class in groups (if a small class) or

2) Give them a task (ftp their site) to perform at home for review at the next class

It is annoying, but as indicated above, it is likely to detect DOS or other malevolent connections.