What actually motivated me to do more work in high school was being involved in drama. I enjoyed being part of putting on plays (tech or acting). The extra time between school end and rehearsal was easy to fill with homework. My grades went up.
I was doing this behind my dad's back though, because he thought being allowed to participate in drama should be the reward for getting good grades (or being forbidden the punishment for mediocre). He found out. Pulled me out of tech on the play. My grades went back down because I would just go strait home and spend my time reading or playing on the computer.
I was tagged as gifted in 1971 and it there was no real procedures to make the best of the situation. I was merely bumped a grade and left to handle myself. My social development took a hit as I was suddenly "better" than my old friends and some uppity little kid to the new crowd.
Then we moved to a different state and the rules did not recognize the grade advancement and I had to repeat 5th grade. Bored and frustrated would be an understatement. In the 10th grade I found some teachers that really pushed me.
You may have the toughest job at home! You may find yourself in conversations or asked questions that are beyond your knowledge. I taught a nephew Latin while we studied chemistry while trying to tuck some ratios and percentages into the mix.
It is simple to treat kids like they are dumb. Treat them like they smart yet uninformed.
Bruce