This robot has been designed for the VEX-IQ robotics kit. It can be made from parts from a single VEX-IQ Starter Kit (http://www.vexrobotics.com/vexiq/products/starter-kit-with-sensors.html)
This design is quick and very simple, it should take no more than 10 minutes depending on your students. It is not the most robust design, but is intended to be built quickly by students, allowing them to spend more time with the programming aspect of STEM education. There are plenty of connection points to add sensors and make the robot more sturdy, and I highly encourage you to get your students to come up with their own modifications.
For a faster build, have one student do the left motor, one do the right motor and one do the castor. Then bring them all together at the end.
If you do use this in class, please let me know, I love to see how far and wide my designs go :)