。
By 8 tomorrow morning, they'll already have gone to school.
Literal
Tomorrow-morning 8-o'clock [until-までには] they [topic-は] school [to-に] go [completed-てしまっている] probably.
行ってしまっている combines 行く + ~てしまう (completion/regret) + ~ている (resulting state). The result: 'have already gone (and are in the state of having gone).' Japanese's aspect system lets you layer completion, resultant state, and probability in a single verb cluster. までには emphasizes the deadline with contrastive は.