Hi Hingol,
I don't think there is a single straight forward answer...You need to match the hatch.
If they are schooling, then they are feeding on something. Try and find out what they are feeding on and then match your lure to what they are eating. Maybe some one near by can fills us in on what the thread fin are feeding on.
Otherwise, try different lures until you get a strike and then stick with that. If they are schooling they might be feeding at different levels. So try some soft plastics along the bottom right through to floating poppers on the surface and some bibbed divers in between.
Without knowing what they are feeding on, I cannot think of what else to suggest.
itsaboat mate...Life is just a boat and then ya marry one !