Port 81 is the "default" port (if not using port 80) in the config file for lighttpd and for that reason the above instructions suggest using port 81 for lighttpd. However it only takes very simple edits to change that.
Some people think that port 81 is an alternate port for http. See e.g.
where they state
WebServer Alternate Port 81
When a user types in your IP address or Host Name in their browser, the browser will look at the IP address on port 80. If the Port is blocked, port 81 is used as an alternate port for hosting a website.
Some sites block high level ports, some block all but a very restricted set of ports, etc. If students are blocked from accessing stuff on port 81, I would first suggest trying to have the IT people open access to requests on port 81. If you have control of the WeBWorK server, you can configure lighttpd (any the associated WeBWorK requests) to use any available port. For example port 8080 if that is not blocked.
If students can only access stuff on port 80, then one can forgo the use of lighttpd (which will put a slightly heavier load on the server) and this can be done either system wide (change global.conf) or on a course by course basis be editing the course.conf file.