Friends

Sorry, this is a learning platform, not a social media site.

Every time people ask for features like that, Khan Academy says that's not what the site is for.  (It would also be extra work for Khan Academy to respond any of those posts that were inappropriate -- such as bullying.  Khan Academy already spends a lot of effort responding to inappropriate posts in the Discussion area of the courses.)

You can create a texting group or an email group or a private group on a social media site and invite your classmates to it.

~ Janice, KA user

But they could set up learning systems, eg streaks, Kahoot-style quizzes, etc without any posting systems

and they could have a system where the person has to send friend invitations and both people (and/or a guardian) have to accept the request.

mochi

It would be extra work for Khan Academy to respond to any of those posts that were inappropriate -- such as bullying.  Khan Academy already spends a lot of effort responding to inappropriate posts in the Discussion area of the courses.
That's probably why every time Khan Academy responds to a request like this one, they say that friends and chatting are not what the site is for.