[This question was answered by a parent from Doug's ministry]
It sounds like you have a very common problem. First of all, addressing it and staying involved with these students when it would be easier to focus on others is a great beginning. It models that you are striving to be like Christ who spent a great deal of his time on earth with those infirmed, broken, and abandoned by society.
It also sounds like you have taken the time to feel empathetic for those who most desperately need fellowship and friendships. Your continuing to model respect for them and valuing them even when they are frustrating will speak volumes to the other students you are discipling. I am involved in disability ministry and when behaviors get in the way of the goals of the group we always put the person first by bringing in added support. We make sure that a person with a disruptive behavior has one on one support and we remind them what is appropriate in a way that they can maintain their dignity. This can be done by alerting mature kids in advance that you want to model Christ's love be being inclusive and that you will need their support to do so. You can rotate assigning support from within the group or recruit a volunteer from outside of the group to focus on that person (the cooler that person is the easier it is to get buy in from the group). The leader of the group will set the tone and his or her tone will be the most influential.
You are so wise to address this problem as allowing inappropriate behaviors that supress the rest of the group will in the end hurt everyone causing typical student's attendance to decline and the student with special needs will be robbed of the friendships they so deserve.
If you are looking for written resources anything written by Henri Nouwen is a good start.
Feel free to contact me.