With a serial LCD backpack the chip that runs the LCD is "attached" to the LCD. Parallel LCDs use at least six pins from a MCU so a lot of times the project MCU does not have enough spare pins.
Use one pin from the project MCU to send data to the LCD MCU serially.

Are suggesting that all the students' cars contain standard PBP code (one license).
Yes, base code written by you.

To experiment with sonar calibration and steering reaction time relative to changes in car velocity and 'race' course size each student has a set of definitions that can be downloaded via MPLAB or PIC kit 2 programmer?
Yes again, these commands will be downloaded to a second MCU that communicates with the main MCU that you programmed.

The students will be able to use the free demo version and not be able to "mess up" the main chip.