Hoping to keep teachers and police officers from fleeing San Francisco because of its high cost of living, city officials unveiled a program Friday that offers down-payment assistance for the two professions. Credentialed teachers employed by the San Francisco Unified School District who are purchasing their first home in the city could receive a $20,000 interest-free loan to help cover the cost of a down payment. Teachers whose households earn less than 200 percent of the area median income, roughly $120,000, would be eligible. If teachers remain employed in the district for 10 years, the loan will be completely forgiven.