Complex sentence: At school, he was both quiet and timid; on his first grade report card, one teacher described Dahmer as a reserved child whom she sensed to feel neglected, and he was regarded as a "quiet kid" by many of his peers.
Split into simpler sentences: At school, he was observed to be both quiet and timid; one first grade teacher noted upon Dahmer's first grade report card that she observed him to be a reserved child whom she sensed to feel neglected. Nonetheless, he was regarded as a "quiet kid" by many of his peers.