The risks you run by riding a single file line:
1. cars approaching from behind may feel like they can "thread the needle" between your line and oncoming cars. This leaves you no margin for error.
2. if there's a side wind that causes riders to echelon out into the lane, you'll be wider than a double paceline.
I think that the more real estate your group takes up, the more you discourage motorists from flying by you. Unfortunately, you piss off everybody in the county.
Check your local ordinances. If the bike lane is adjacent/attached to the roadway, you're likely obligated to use it. If it's more of a wide sidewalk away from the road, it's probably optional.
btw, In Michigan, you can ride on the road regardless of a bike lane if you're 18 or older.