Maybe it was a giant house cat. Maybe it was a bobcat. But that tail—that long, thick tail—that tail said mountain lion. It was Wednesday, Jan. 28, at 3:30 p.m. and Victoria Canby had just finished teaching an outdoor art class to fourth and fifth graders at West Marin School. Seated in her car, she checked a few texts before her drive home to San Anselmo. Then she saw the cat on the other side of Highway 1, directly across from the school. “When I saw the tail, that’s when I knew for sure,” Ms. Canby said. “Oh, my gosh, it’s a mountain lion. What if there are kids in this parking lot?” There were no kids in sight, but Liz Wilhelm and Taira Restar, two outdoor educators who had also just finished teaching, were on the scene. “Hey, guys, did that look like a mountain lion?” Ms. Canby yelled across the parking lot. Ms. Restar didn’t get a clear look, but Ms. Wilhelm felt sure. “That’s a mountain lion,” she told Ms. Canby. They watched the cat disappear behind a house and seemingly leave the area. A snoozing dog across the street was unperturbed, indicating that no threat lurked nearby. The women conferred for a moment about what to do. They decided not to sow panic, and they went home. But Ms. Canby couldn’t get the cat out of her mind. “The kids are like little snacks to them,” she said. “I didn’t want anything to happen to anybody. It would be so tragic.” A couple hours later, she called her friend Claire Burns and told her what she had seen. Ms. Burns, whose two sons are students at the school, marched over to the sheriff’s office to report the incident. “We did high-visibility patrols throughout the night and for the next couple of days, just in case it was to resurface,” said Deputy Jason Swift, who was at the school early the next morning, before everyone except the maintenance man had arrived. The school sent out an email warning parents, but no one panicked. After all, West Marin School is the Home of the Wildcats. Mountain lions, also called cougars or pumas, can leap as far as 45 feet in a single jump. They weigh as much as 175 pounds and their long tails help them with balance. When sprinting, they can hit 50 miles per hour. But cougars are relatively timid cats who avoid human beings, according to Quinton Martins, director of True Wild Conservation, a nonprofit that studies lions in the North Bay. Attacks on humans are extremely rare. True Wild has counted 75 mountain lions living in Sonoma County, where it has been studying the cats for a decade. In Marin, where the group only began working last year, Mr. Martins estimates there may be only six to 10 living year-round. Two summers ago, an Inverness mother and son spotted two of them snarling at one another in their backyard tree—perhaps brawling over turf. If you see one from a distance, enjoy the sighting, Mr. Martins advised. If you have a close encounter, raise your arms, act big and make noise. Grab a rock or a stick, if any are handy. “Never turn your back and run,” he said.