Privacy error / certificate is not valid

Hello! I've been having an issue with my mac.

I am running El Capitan 10.11.6. There are no updates in the app store so I assume my computer is up to date.

I've been having an issue where when I go to certain websites it brings up a "privacy error" below:

This same error occurs in any browser I use including chrome, safari, and firefox. I also know that this problem has to do with my computer because the same websites I try and access work fine on my laptop and phone under the same internet network.

I looked up a bit on how to solve this problem and from what I found it appear to be an expired root certificate?

I was wondering how I fix this? I was also wondering if my computer is at risk because of this issue? I'm not going to any sketchy websites, the example used above is just my school website I've been using for years on this computer. I can still access the website by bypassing it, but I'm noticing this issue is having bigger issues on other websites (like images not appearing/loading). Any help would be appreciated!

edit: also wanted to note that I've tried restarting my computer, updating chrome, and clearing my cache/history so far to try and solve this and none of those have worked