Why did the girls smirk so much?

If they did smirk without an obvious catalyst it would've been the older girls doing so at their predicament. Teenaged girls living the cloistered life throughout their high school years wasn't (and isn't) as archaic as popular culture may have led you to believe (it's much more common for girls 16 and under). The girls for whatever reason apparently never raised a fuss about it and for that matter, weren't the instigators of the basement party. The doctor's prescription, the remedy, for their dreamy little sister's histrionics over an unrequited crush was to be "codification" by boys, with the dosage to be administered at a basement party. To the cruder mind the narrative would've couched it as oppressed catholic girls following doctor's orders to get some, and indeed, down the road, the neophyte slut Lux said she "needed to have it."

This is the image of Lux during the basement party, this particular unguarded smile is mentioned in the book:

http://screenmusings.org/TheVirginSuicides/pages/TVS_149.htm

A devotee of fashion magazines, Mary cultivated a sophisticated veneer that responded to the indignity of the teenaged experience with a cynical smile. Her greeting at the basement party wasn't flourished by a smile as much as the musculature needed to keep a snide remark bottled up.

A smirk can convey a wide array of emotions. Were they smirking during their clandestine flirtations with Peter Sissen when he was over for dinner?

The boys invited to the basement party were from the Lisbon's neighborhood, one from as close as across the street. Though they may have genuinely cared about the sisters, they were as a group the sort of "sunken chested" wimps Mr Lisbon had hoped his daughters could avoid. On the arms of the football squad at the Homecoming Dance the only discordant expression that flashes across any of the girls' faces is perhaps Bonnie's glare as she watches Lux protectively.

Therese in the foreground of Mrs. Lisbon's chastisement of Lux for footshow:

http://screenmusings.org/TheVirginSuicides/pages/TVS_480.htm

Tim's snores are criticized from the afterlife:

http://screenmusings.org/TheVirginSuicides/pages/TVS_305.htm

I haven't read the book but considering the film and everything we see in it is through the boys filtered lens, perhaps the smirks were just another indicator of the girls knowing something the boys didn't..or at least the boys remembering it that way, as years later they still cannot work out the full story. If that's the case, it just makes Sofia Coppola seem even more brilliant imo for adding such a subtle element.

Ahhh, boys. Y'all are boys. A lot can be hidden in a "smirk". Clearly you've never been a 13 year old girl. ;)