Do some websites block school building/classroom IP networks because they mistake us for possible DDOS? So I had my students go to NodeJS.org to download Node.js, and they did it with no problems. Two days later, I had them go to NodeJS.org again, but none of us could access the site, not even from the designated "teacher's computer" of the classroom I was using.

However my laptop, which was connected to the school's wireless service for teachers, could still access the site. I'm assuming classroom computers and the school wireless are on separate networks. Could this mean that the anti-DDos system at NodeJS.org basically mistook us for a potential DDoS and blocked us? There were about 20 students. A week later we could access the site again.

So I'm just wondering if anyone else experienced anything like this?
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.