Melania Trump says that kindness, compassion and positivity are important traits in life.
The US first lady helped SADD - Students Against Destructive Decisions - open its annual national conference at a hotel just outside Washington.
The appearance followed her trip last week to the US-Mexico border to see the effect on children of her husband's "zero-tolerance" policy against illegal immigration.
"Kindness, compassion and positivity are very important traits in life," she Mrs Trump said.
"It is far easier to say nothing than it is to speak words of kindness. It is easier to judge quickly than to take time to understand. It is often easier to see a glass half empty rather than half full."
"Nevertheless, you have the power to be the positive force in so many people's lives," added the first lady, who is the mother of a 12-year-old boy.
"Show respect to each other. Treat your community like your family, and look out for one another."
SADD was founded in 1981 as Students Against Drunk Driving but changed its name and mission in the late 1990s to focus on prevention of all behaviours and attitudes that are harmful to middle, high school and college students, including substance abuse, traffic safety and bullying.
Mrs Trump said she was "inspired" when she first heard about the organisation and the conference.
Before she addressed the gathering, Mrs Trump met with the organisation's leadership and helped make blankets the group is donating to shelters, said her spokeswoman Stephanie Grisham.
As audience members awaited the appearance, they rocked out in their seats to music by Justin Bieber, Neil Diamond and Journey.
