Group co-operation is taught wonderfully in the home first. Daily chores or errands as well as new tasks can be accomplished with co-operation from all household members. Later this ability can be polished in child focussed groups such as scouts, guides, bible group, etc. Friendships aren't only made in schools. Friends are gained through time and effort during meaningful and interesting interactions.
Homeschooled children learn safer, more effective ways of dealing with threatening situations.
Children who feel the need to learn how to defend themselves physically can enroll in a martial arts program. Being around adults setting good examples of manners is far better than being around peers who don't have any. Manners are best taught in the home, with loving guidance, rather than in the schools with shame. In the community, when they are with us all day. At least this way we can instill our values into our young charges during the impressionable years.