A new survey finds an estimated 17 percent of American high school students say they drink, smoke or use drugs during the school day. The National Center on Addiction and Substance Abuse (CASA) at Columbia University found 86 percent of teens say they know which of their peers are abusing substances at school, CNN reports.

The findings come from an annual telephone survey of about 1,000 students ages 12 to 17. According to the survey, 60 percent of high school students say drugs are available on school grounds, and 44 percent know a classmate who sells drugs at school. Marijuana is the most commonly sold drug at school. Prescription drugs, cocaine and Ecstasy are also available.

Social media plays a role in peer pressure to use drugs and alcohol, the study found. Three-quarters of students said they are encouraged to use marijuana or alcohol when they see images of their peers doing so. The survey found 45 percent said they have seen photos online of their classmates drinking, using drugs or passing out, up 5 percent since last year.

For the first time in the history of the survey, a majority of private school students—54 percent—said their school was “drug-infected.” In 2011, that figure was 36 percent.

Teens are more likely to use drugs or alcohol if they have been left alone overnight, and are less likely to do so if they regularly attend religious services, the survey found.

“The take away from this survey for parents is to talk to their children and get engaged in their children’s lives,” Emily Feinstein, project director of the teen survey, said in a news release. “They should ask their children what they’re seeing at school and online. It takes a teen to know what’s going on in the teen world, but it takes parents to help their children navigate that world.”