SANTA CRUZ DE LA PALMA, Canary Islands—Authorities on the Spanish island of La Palma are telling people who live near an erupting volcano to stay indoors because of a heavy fall of ash that has forced the cancellation of flights and school classes. The Cumbre Vieja volcano on La Palma, which is part of Spain’s Canary Islands off northwest Africa, has been spewing lava, ash, and gases for more than six weeks. The eruption has alternately surged and ebbed since Sept. 19. Local air quality is “extremely unfavorable” because of high levels of small particles in the air, emergency services belonging to the Canary Islands government said in a statement late Tuesday. All flights to and from the island have been canceled because of the falling ash, according to Spain’s national airport authority. With flights canceled, some tourists who came on a sightseeing trip to witness the eruption had to …