What if I told you I think of you often? Would it make you feel uncomfortable?
What if I told you your hair style is one of my favorites? Would it make you feel uncomfortable?
What if I simply said, “You look great?” Would it make you feel uncomfortable?
What if I asked how things are going for you these days? Would it make you feel uncomfortable?
Now, when it come to explicit imagery of genetalia and discussion of such things between strangers, *that* ought to make any normal person uncomfortable at best.
It should be kept strictly professional.
Interaction between equals or co-workers is something different. But between teachers and students the balance of power is so very unequal that keeping things professional is the best way to go about it.
PS the answers are yes, yes, it depends and it depends.
Teachers should not be messaging their students on snap chat under any circumstances! Snap chat is not where class assignments get posted.