"Server Identity" issues on my iPhone Calender app.

When I'm at school my phone would normally have wifi so I could talk to my friends and also watch videos. Three days ago I wasn't able to anymore. My phone would get a message saying "cannot verify sever identity, the identity of "calendar.google.com" cannot be verified.". but I don't have any google calendar apps and ive closed all my tabs. Ive also tried restarting my phone. Any solutions?

[Re-Titled By Moderator]