I have been thinking about your little attention-getter. I am glad that my strategy for ignoring him is working enough so that you are able to get through a lesson. However, it is important that you find things that the attention-getter really wants. Does he like being the line leader? Does he enjoy time on the computer? Does he like to play Leggos? Find out as much about this child as you can so that you can set up an intervention that uses something that he really wants as a reinforcer. He needs to earn points or checks or something that has a pay-off for something he really wants. At the same time, start looking for ways to praise the child for appropriate choices - with check, with stickers, with verbal praise and your attention. He wants attention so find ways to give it to him in a positive way. It'll take a lot of your time in the beginning, but it took years for the child to get this out of control, so it will be awhile before he will turn around. Stay the course.
In the meantime find someone at your school that can come in and observe this youngster so that you have a partner at your school that can help as you deal with the ups and downs. Let us know how it goes.