We are thinking that this may have been accessed at home first then forgot to possibly close out and the phone auto connected to our network thus causing the filter trip. What would you all do?
If your network filter caught and prevented the content from displaying, and your logs can support the idea that this was an honest mistake and not an attempt to browse porn at work, why is this even a problem? Content blocked. No porn was accessed.
However, IT needs to comply with the administration's policies in this. It's not up to you to decide what gets reported and what doesn't. (Unless, of course, it is up to you.) If the policy mandates all access attempts are reported to school administrators and investigated, that's what you do. If the policy doesn't specify, and the log data supports the premise that it was an honest mistake caused by an open browser connection and a phone auto-connecting to the school's WiFi, I'd provide the data & your professional opinion to your department supervisor and let them decide whether and how to bring it up with the user.
Speaking as an IT professional who's dealt with similar situations in the past and in the absence of policies governing how to proceed, I'd approach the user after school and off school grounds (or at least in the parking lot) and say something like, "Heya - a quick reminder that all network traffic on the staff WiFi is logged. If your phone connects automatically, it's a good idea to make sure you've properly exited whatever apps you use at home before you come to work."
Pause for a moment while giving them a meaningful look.
"Have a great night!"
They'll probably get the hint.