Bullying is not a natural part of growing up and should be recognized as a major public health problem with consequences lasting far beyond childhood, according to experts in a new report by the National Academies of Sciences, Engineering and Medicine.   Parents and teachers have traditionally brushed off complaints of bullying, telling victims that they must fight back or that they brought the abuse on themselves through their own behavior.   But the lead author of the report, pediatrics professor Frederic Rivara, says bullying has "lasting negative consequences and cannot simply be ignored."   The study says youngsters who are pushed around by bigger or older children can suffer from poor grades in school, anxiety, depression and a higher suicide risk — and that the bullies themselves can develop the same problems.   Bullying has moved beyond the playground and into 21st-century technology with cyberbullying in internet chat rooms, on Twitter and on other social media, the report says.   The experts recommend that schools give up their zero-tolerance policies, where bullies are automatically suspended from classes, saying those policies do not help.   Educators and others must try to understand the context in which bullying occurs and undertake so-called preventative intervention policies, according to experts.