。
She was shy in high school.
Literal
She [topic-は] high-school-era shy was.
高校時代 ('high school era / days') uses 時代 ('era, period') as a productive temporal suffix — 学生時代 ('student days'), 江戸時代 ('Edo period'), 子供時代 ('childhood'). The bare noun functions adverbially as a time frame; no particle is needed. 内気 ('shy, reserved') is a compound of 内 ('inside') + 気 ('spirit, mood') — a temperament that keeps emotion turned inward. The opposite is 外向的 ('extroverted'). Japanese has a rich vocabulary for personality types arranged on the introvert/extrovert axis, with 内気 sitting on the gentle end of reservation rather than active social anxiety.