Maybe you could try some "get to know you" type activities to help the students connect on a deeper level than just the criteria which is causing the "outcast" child. Once students see that they have things in common with the child they repeatedly leave out of activities, it will be easier for them to accept the child.
One of the games I play at the beginning of the year is a classroom scavenger hunt. I have a list of items on the paper such as:
Find someone who has a brother/sister, who was born in another state, who has been out of the country, who likes broccoli, who is allergic to something, who has a pet, who speaks another language, etc.
You can make up as many things as you need, or look online for similar "teamwork" type activities.