I lost my Iphone at school and it is not connected to find my iphone. Which is the cheapest and easiest way to find my phone?

I don't know which method is the best from this link: https://www.lifewire.com/find-a-stolen-or-lost-iphone-2000297

and also, will the life360 app work to find my phone or would I had to sign up for it before my phone got stolen?

You're out of luck.

If you never signed up for any service like iCloud, you won't be able to track it.

Though, the NSA probably already knows where your phone is…

Any app or service, you would need to have installed and activated at the time the phone was lost. As far as I know, there are no after the fact, apps that can be remotely downloaded or turned on, to locate and iPhone.