A continuous rotation servo is a servo that does not have a limit on its range of motion. Instead of having the input signal determine which position the servo should rotate to, the continuous rotation servo relates the input to the speed of the output and direction. For example, an input PWM signal of 1.5 ms relates to the center position, so the servo will not move. A PWM signal of 1.0 will turn the servo full-speed in the clockwise direction and a PWM signal of 2.0 will turn it full speed in the counter-clockwise direction. Values in between 1.0 ms and 2.0 ms will turn the servo at corresponding speeds and directions. Source: CMU.edu

Extra ideas for more precise servo motor control

Have the students figure out how to change the speed of the servo motor moving from position to position and maybe try making a clock with multiple servo motors running at different speeds.