He has flippers instead of feet — and certainly no sneakers or hiking boots. But that didn’t stop a sea lion from joining schoolchildren on a walk-a-thon. The marine mammal apparently noticed children doing laps Friday morning around a course they had set up at the Marin Country Day School next to the shores of the San Francisco Bay. The 185-pound Steller sea lion waddled ashore, shocking students and teachers. “He did a whole lap,” said Kelly Watson, director of constituent relations and web communications at the private school. It was the latest brush with humans for the 1-year-old sea lion, called Astro by staffers at the Marin Headlands-based Marine Mammal Center. Astro’s mother abandoned him at Ano Nuevo Island off the San Mateo coast in June, prompting biologists to bottle-feed the pup. They released the adolescent on April 25 with a radio tag. But Astro keeps returning to civilization. About a week ago, he swam under the Golden Gate Bridge to the shores of Corte Madera. The Marine Mammal Center again picked him up and released him in the Farallons, 27 miles from San Francisco. But returned again Friday, just in time for the walk-a-thon.