Tests

It would be great if teachers can implement a group and individual assessment. For example, the first part of the test can be in groups where each group is asked to work together and construct a model that explains the relationship between each of the components of a circle. Additionally they can accompany their model with a paragraph explaining those relationship. Then after that is done they can work individually to complete a worksheet with different radii and solve using the different formulas.

Formative Assessment

As a teacher it is important to see what the student has understood from the lesson you have taught. In order to see if the previous paper plate exercise worked, it would be great to have some type of formative assessment. One way you can do this is by having the students individually fill a chart that asks these questions: What do you think you know? What do you understand very well? What don't you understand? What would you like to review?
This will be a great tool to get an overall feel if the class understood the material.

Circle & Area/Circumference

One way to expand on the paper plate exercise, the teacher explain the area and circumference formula and how they relate to each other. Then, they can ask students to work together in groups and provide them with different sized radii and ask them to solve using the formulas.

Circles and Paper Plates

It is really cool when students can have hands on activities to learn about circles.
One way you can help students learn about radius, diameter and circumference is by letting them label the parts of a circle on a paper plate.
This way students can remember where they belong on a circle and the teacher can explain the reason they are displayed that way.