People effected by the Saugus High School shooting leave candles and flowers during vigil at Central Park in Santa Clarita on Thursday. Photo: IC

A teenage boy gunned down fellow students at a California high school on his 16th birthday Thursday, killing two and wounding another three before turning the pistol on himself.

The gunman was in "grave" condition, police said, as officers stormed Saugus High School in Santa Clarita - the latest in a relentless cycle of classroom shootings in the US that have left around 300 youngsters dead over two decades.