[aninhumer]

Guaranteed? What makes you so sure? And then you need to find a power socket you can use (not difficult in a school, but could be elsewhere), and wait for the minimum charge level, and then wait for it to boot.

But if you know the number, you can use your friend's phone in a few seconds.

[dpark]

Same reason you're sure that one of your friends will have a cellphone. The numbers make it exceedingly likely.