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She was very shy until she graduated.
Literal
She [topic-は] graduate [until-まで] very shy was.
卒業するまで ('until graduating') uses まで ('until') after a dictionary-form verb to mark the time-bound during which the state held. The verb stays in non-past form even though the main clause is past — Japanese clause tense is relative to the main clause, so まで with the dictionary form means 'up until the moment of doing X.' 内気 ('shy, bashful, reserved') is a na-adjective for an introverted personality, slightly more poetic than the everyday 恥ずかしがり ('one who shows shame').