You need a smartphone to take part in modern social life. (Yes, adults can choose life without a smartphone or any social media, in reality teenagers can't without being ostracised).
On the other hand using smartphones and social media can also result in stress and comparing your ordinary life to other people's highlights.
Also, there are tons of things that have ostracized teenagers in the last. Why do smartphones have such a stranglehold? It seems we are becoming more and more afraid of being different.
Without a smartphone and access to the chat app of choice, you'll be left out of a good part of out-of-school socialisation. For example nobody gives out invites to birthdays on cards at class anymore, they do it via WhatsApp and you reply the same way.