Playmates
industrious angels
The oxymoron matters—angels are usually ethereal beings, but Dickinson gives them work schedules and recess breaks, making heaven weirdly bureaucratic.
forgot my school-mates
The dash creates a dramatic pause before the abandonment. She doesn't just prefer the angel—she completely erases her human friends from memory.
How dreary marbles
Marbles was a common children's game in 1860s New England. The shift from heavenly "Crown" (likely a royal game) to earthly marbles shows the permanent downgrade after transcendent experience.