If the first element of art is line and the second element is shape, the next logical element is form.
Line is one dimensional. Shape has two dimensions. Form is created when a shape takes on three dimensions. The three-dimensional character of form can be real, as in a sculpture. Or it can be implied, as in a 3-D drawing.
Here are some great web pages and tutorials that will help you understand form and how it works in art.
K-4 LESSON PLAN
Here is a complete lesson plan from the national gallery of art: http://www.nga.gov/content/ngaweb/education/teachers/lessons-activities/elements-of-art/form.html
This short page is subtitled Notes and Examples of How to Illustrate Form. It provides a concise resource for young artists who want to explore form: https://www.smore.com/efkd-elements-of-art-form
FOR OLDER STUDENTS
This page is a bit more involved but will be a valuable resource for high school age students who want to dig deeper into this element of art: http://visualartspdsf.blogspot.com/2012/04/form-or-volume.html
The following short (3 min.) video shows how form works in art and sculpture: