“Large portions of US teens will get second phones for one specific social media app” is an absolutely wild thing to predict seriously
I'm not saying it's more likely that not, but I am saying I wouldn't be surprised. If you replace "one specific social media app" with "iMessage", it has already happened.
I don’t think the average American high school student has two smart phones one of which is a secret from their major source of income (their parents).
There's no use for a cellphone plan, we would just use wifi or hotspot from their main phone.