The sunny weather Thursday may be putting San Franciscans in a good mood, but a group of students at a Sunset District middle school are being told to slap a grin on their faces as "Smile Day" is under way.

As part of the San Francisco Unified School District's "Respect Week," students at A.P. Giannini Middle School are being asked to show off their pearly whites.

"The reason 'Smile Day' is important is because people seem approachable and friendly, and it's much easier to get along when someone appears friendly," district spokeswoman Heidi Anderson said.

Throughout the day, undercover "smile spies" will be in the hallways watching for grins and surprising students with "I got caught smiling" stickers, Anderson said.

At lunchtime, students were going to examine close-up photos of the mouths of various teachers and staff members, trying to guess their identities from their smiles. Those who guess correctly get a prize, Anderson said.

Respect Week, which coincided with Martin Luther King Jr. Day, is part of Violence Prevention Month, in which the district focuses on topics including tolerance, conflict resolution and bullying prevention.