Complex sentence: Walter Jr. trained as an engineer at the Mechanical Arts High School in Boston, but he was artistically inclined and upon graduating from there in 1912, proceeded to the Massachusetts Normal Arts School, majoring in painting, also studying architectural drawing and American history.
Split into simpler sentences: Walter Jr. trained as an engineer at the Mechanical Arts High School in Boston, but he was artistically inclined. Upon graduating in 1912, he proceeded to the Massachusetts Normal Arts School, majoring in painting while also studying architectural drawing and American history.