Translation:The parents are advised through the school.
That's just how the passive voice works.
Ok, so here "durch" gives it away. But I was looking at it, and not knowing the passive, I only saw a future there. I deduced it should be a passive because it was the only meaning that made sense. In general, if we don't have a durch, how can we distinguish them? only by context?
Yes, you'd have to go by context. This sentence is a bit of an exception, though. Usually, the participle is not the same as the infinitive.