What if they are related? (the mother/father/older sibling is the child's teacher)
What if they have known eachother from before entering the school setting (the teacher is the student's close friend)
What if they are homeschooled?
What if the teacher uses said social media for convenience sake (i. E helps the student during non-work hours / sick leave with the material currently studied).
--On a personal level--
The slight probability of a power dynamic forming is highly outweighed by the benefits of such relationship.