Those warnings are only shown for users with permission to view problem debugging info as stated in the message. So your students will not see those.
We changed the way that those warnings are handled to make these warnings more visible so that problem authors will see issues with their code and start fixing them. Previously these warnings only went to the apache2 error log.
In this case, you have discovered a minor issue with the openDivSpan method defined in PGbasicmacros.pl. The $html_attribs is defined without a default value. So if no html attributes are passed in the options, then it remains undefined, and when it is used later it gives the warnings. I will open a pull request to fix the issue.