If I say
I've never driven from coast to coast.
the intended, and understood, meaning is "from the Atlantic to the Pacific", assuming the conversation has the USA as context.
Only a non-native speaker who is learning English might say
I've never driven from the coast to the coast. unidiomatic
Why do native speakers omit the article there?
Because we're not referring to coast as the focus of an utterance about a particular coast; we're using coast as member of a dyad, a pairing, expressing the boundaries of some kind of extent, analogous to "from top to bottom" or "from dawn to dusk" or "from side to side" or "from edge to edge" or "from bow to stern".
The same goes for from sea to sea.
Here's a sentence showing both the extent-dyad and the focus-of-utterance:
I looked the statuette over from top to bottom. There was a small chip at the top and the bottom bore a maker's mark.
P.S. The teacher-student dyad can be understood as the two endpoints of a relationship, or they can be understood as roles. When referring to a role (as distinct from a particular instance of a person in that role) we omit the article.