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The boy was forced to drop out of music school at twelve.
Literal
Boy [topic-は] twelve-years [at-で] music-school [object-を] drop-out-[causative-passive-させられた].
The causative-passive ~させられる is a notoriously tricky compound auxiliary meaning 'to be made to do X (against one's will).' It stacks the causative (make someone do X) with the passive (be on the receiving end), yielding 'be forced to X.' The particle で ('at') marks both the age (at 12 years old) and location (at school) — in this sentence, age. 退学する ('to drop out / leave school') takes を for the school — 退学する is a nominal-verb compound.