[bread_baker]

Try to plan the preceding classes so that they are dismissed on time. That will help a little.
I'd reward the strong and eager students by making them group leaders. If you try out a new activity with the leaders, give them a handout and go over it if possible. Practice the activity with the leaders. This way you know they understand it and can help the other students.
You may have to choose activities that are easy, just so all the students understand. In the future you can move up to ones that are a little more difficult.
If the leaders meet with you on their own time, you should reward them in some way. Make sure you thank them every time and make them feel appreciated. Praise them when they do a good job. For some people, this will mean so much to them that they'll do anything for you!