[aninhumer]

And if mobile phones were 100% guaranteed to be charged when you need them, that would be fine, but since they're not, asking a child to memorise a few digits so they're able to use someone else's phone in an emergency is perfectly reasonable redundancy.

Also, as far as I know, there wasn't ever a time when people knew how to send telegrams, but didn't know how to write letters. Indeed most people didn't know how to send telegrams with a telegraph key at all, they'd just write up their message and take it to an office to be keyed by an operator. A process which is largely identical to posting a letter.

[dpark]

You know you can recharge a cellphone, right? In fact you can make a call while it's still charging! Guaranteed one of your friends has a charger you can use.

[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.