In 1999, teen culture underwent quite a dramatic shift.  Brighter, more clean cut fashion replaced much of the darker, shock-rock inspired fashion of the mid '90s.  Things like oversized wide-leg pants like JNCOs, oversized shirts with some rock band logo, trench coats, etc made way for cargo pants and Abercrombie and other logo t-shirts.  Shock-rock itself began to decline in favor of teen pop, R&B, and hip-hop.  Goth was on its way out.  There was definitely a shift from a darker, more mature culture to a more lighthearted one.  This was also around the time that bullying began to be taken more seriously in schools.

The fact that this occurred primarily in the summer of 1999 makes me think about the shift's relation to the Columbine shooting.  Much of what was mid '90s culture was blamed for the shootings.  Stuff like Marilyn Manson and other shock rock and dark, satanic-inspired fashion.  In 1998 the more rebellious you were, the cooler you were.  By 2000, it had become more about conformity (if there was one way to describe culture in the early 2000s it is "conformity.")

Do you think this shift was already under way despite the shooting or did the shooting precipitate it?

It was really moreso of the industrial rock stuff that got blamed the most (Rammstein, KMFDM, and Marilyn Manson were probably the ones in the radar most). Not to mention, Marilyn Manson was blamed for about 36 other school shootings, some of which were before the ones at Columbine (a big example was Thurston in May 1998 where the perpetrator, Kip Kinkel, also wore a trench coat and listened to shock rock/goth/industrial metal).

All in all, I do agree that there seems to be a "more conformist" society as a result of the shootings, and it also put Generation X culture 36 feet under for sure. I also thought that music began to be targeted towards "a younger audience". There were even toys that got controversy (a WWII toy of a soldier wearing a trench coat was one of them). "Zero tolerance" policies also tightened even more as a result of it.

However, all of this is just based on my research unfortunately.

The booming economy, in the United States, probably overshadowed Columbine, no matter how bad Columbine was. From 1995/1996 to 2000, there was a feeling of euphoria, not just economically, but socially, pop culturally, and energetically, in the United States.

The horror genre had a comeback in December 1996 with Scream. But even that wasn't because of anything bad in the news. It was just a very bouncy, up, up, up, up, and optimistic, time, in America. Everything, and anything, felt possible. We were drunk on optimism. We were probably even a little too confident, but who can blame us, fully, because why be negative in an upward spiral?