Final answer:
Arnold Schoenberg's atonal-Expressionist era reached its zenith with his development of the twelve-tone technique, a groundbreaking change to traditional music composition that continues to influence avant-garde music.
Step-by-step explanation:
The high point of Arnold Schoenberg's atonal-Expressionist era could arguably be his development of twelve-tone technique, which was a radical departure from traditional western music scales and principles. His experiments with atonality represented a significant moment in modernist music, eschewing the conventional expectations of harmony and melody to explore dissonance and a more abstract form of musical expression. Schoenberg's contributions, while perhaps not embraced by mainstream audiences, remain a profound influence on avant-garde musicians and composers, cementing his legacy as a pioneer of modern music.