pack of galahs
A pack of galahs is a bunch of bumbling idiots who think they're the smartest people in the room but are actually just a pile of shits.
My class is a pack of galahs. We all failed the test but still think we're the best.
The pack of galahs at the party kept arguing about the best pizza toppings.
My brother's mates are a pack of galahs. They all think they're the best at everything.