First-class and High up

First-class adjective - Of the very best kind.

High up and first-class are semantically related In some cases you can replace phrase "High up" with "First-class", this phrases are similar.

High up adverb - At a great altitude.

First-class and high up are semantically related in eminent topic. Sometimes you can use "First-class" instead an adjective phrase "High up", if it concerns topics such as high-ranking.