Some sources say that 1 out of 88 children is autistic. http://www.cdc.gov/features/countingautism/ Autism spectrum disorders include Rett Syndrome and Fragile X Syndrome caused by genetic mutations, classic autism (nonverbal and nonengaging) as well as "high functioning autism" (likely to be above or average intelligence with symptoms of autism and atypical speech that is slow to develop) and Asperger syndrome (marked by developed speech.) http://autism.about.com/od/whatisautism/f/whatishfa.htm
Individuals with Asperger syndrome are developmentally delayed in terms of social skills and may display a one-track mind and rigid repetitive behavior. Despite having a vocabulary often beyond their years, they have difficulty maintaining eye contact (i.e. less than 40-60% of the time) or reading other people's facial cues or tone of voice and appear aloof and disinterested. Because of this, they tend to be bullied as young children and ostracized as older children and as adults, which contribute to the development of depression or anxiety. http://www.aamft.org/imis15/content/Consumer_Updates/Aspergers_Syndrome....
It seems likely to me that the young man who attacked the first graders at the Connecticut school harbored a deep resentment against children of this age, perhaps due to unresolved issues related to bullying. (NB: Asperger syndrome in itself does not predict violence.)
Although he was 20 years old, Adam Lanza may have been functioning at around 14 years old, in terms of social and psychological development.
I think it's a good idea to mainstream children with developmental issues but it's also important to educate school children in general about how to treat those who are "special" with compassion, kindness and patience, so that they can blossom at their own pace. http://www.cnn.com/2012/09/07/health/autistic-kids-bullied-time/index.html
Also, high functioning autistic children and those with Asperger syndrome need an environment where they can continue to grow and develop socially and intellectually after they become too old to remain in high school until they are mature enough to transition to college life which can be too stressful for them at age 18 or 19 or even at the age of 20. http://www.childmind.org/en/posts/articles/2012-4-2-aging-out-when-kids-... If they tend to spend a lot of time at the computer, they can be encouraged to interact with other real people by joining Facebook or other social networking sites that are safe for them to chat or exchange e-mail messages.
Social development in children http://www.scanva.org/prc-social-development-in-children.htm
Child development theories http://psychology.about.com/od/developmentalpsychology/a/childdevtheory.htm
Violence prevention program http://www.violencepreventionworks.org/public/index.page
Added on 12/18: