。
She used to wash her hair before going to school.
Literal
She [topic-は] before-school [before-前に] often hair [object-を] washed [used to-ものだった].
~たものだった is the nostalgic-past pattern — 'used to do X (in the old days).' It carries a reflective, reminiscent tone that plain past tense (洗った) doesn't. Combined with よく ('often'), it paints a habitual past routine. 登校前 uses Sino-Japanese 登校 ('going to school') + 前 ('before') as a compact time expression.