On the wall directly opposite my classroom door hangs one such sign.
Students swelter in my stuffy classroom where temperatures hover between 85 and 95 degrees (no, we don't have air conditioning) and for an hour struggle with geometry or statistics problems. Finally, the bell sounds. BBRRRRIIIINNNNNNG!!! As students gather their belongings and scramble to beat each other out the door, an uplifting message awaits their exit.
In all fairness, the sign originally said "NEVER GIVE UP" but the tape on the first part didn't hold and the custodians swept up the fallen paper and tossed it. Much to my amusement, it took a few days for most people to notice and a couple more for the art department to redo the missing word. In the meantime, I seized the day, not to mention my camera, and snapped a few photos. They still make me smile.