The best way to start a new group is for you to move to an alternate time or curriculum and to leave those who will not come along with you with someone else. Because you are the leader, it is almost certain to that half the group or more will come along with you. Because people are creatures of habit, you are almost guaranteed to have enough people stay behind to form the new core. This is the most effective way of starting new classes I know. Every time we have started a new congregation, we have depended on several groups to make the core of the new congregation.
People follow leaders.
A very effective strategy for a church to grow is to launch multiple congregations meeting in the same building. To do this, the church need several existing groups to move to the new hour. This is a very effective way to start new groups and new congregations. An approach that is a little less radical than starting a whole new congregation is for the teacher to move to a different curriculum. If you can find something that is interesting to half the group, but not to the rest, you are almost certain to create a new group successfully.