。
Because class starts at 8:30.
Literal
Class [topic-は] eight-hours-thirty-minutes [from-から] start [reason-から].
The final から is the 'because' particle — trailed off without a main clause, letting the listener fill in the implicit point ('...so we need to hurry' or '...so come on time'). Note that this sentence contains two distinct から: the first (八時三十分から) is the temporal 'from,' and the second is the reason 'because.' Context disambiguates.