In the past several months, three schools in the San Diego region have revealed the presence of alarming levels of lead in their drinking water. Lead is unsafe at any level and it is especially damaging to children’s brains. Now, San Diego Unified and other school districts across the county are moving to test many more schools. Here’s what we know and what we do not. At La Mirada Elementary School in San Ysidro, samples taken last fall found alarming levels of lead coming from eight drinking fountains.