Is it normal/desirable that school kids are in WhatsApp groups with teachers?

[DavidB.]

Today in the Dutch "news": a high school teacher intended to send a picture of his private parts to his life partner through WhatsApp, but clicked the wrong conversation. The picture ended up in the WhatsApp group of a high school class he taught some subject.

Oops.

Apart from the LOL/WTF factor, I found it astounding that kids nowadays apparently interact with their teachers through their smartphones. I don't consider myself old, but in my time, I found e-mailing with teachers already quite a "low-threshold" activity (and smartphones barely existed). I don't really see the necessity of kids sending WhatsApp messages to or texting with their teachers, nor do I find it particularly appropriate. What's more, kids who don't have a smartphone might miss out on important information everyone should have access to, since apparently, this WhatsApp group was meant to be used for that.

What's your opinion on this?