An affluent resident of an upper middle class town, Lanza had exactly the kind of resources that you would want for taking care of a kid with these kinds of problems. His parents had all the money he needed to get him help, and his school did everything they could to help him cope, according to the Wall Stret Journal: "Not long into his freshman year, Adam Lanza caught the attention of Newtown High School staff members, who assigned him a high-school psychologist, while teachers, counselors and security officers helped monitor the skinny, socially awkward teen, according to a former school official.
"Make more mental health resources available" or "early identification and treatment of troubled children" is a fine answer to many cases, but Adam Lanza had all that you could wish for in terms of resources. It didn't stop him from picking up a gun and going to that school.
What Lanza shows us is the limits of the obvious policy responses. He had all the mental health resources he needed--and he did it anyway. The law stopped him from buying a gun--and he did it anyway. The school had an intercom system aimed at stopping unauthorized entry--and he did it anyway. Any practical, easy-to-implement solution to school shootings that you could propose, along with several that were not at all easy to implement, was already in place. Somehow, Lanza blew through them all.