I agree with Chris's comments.
I do have something to add.
The way I motivate my students which works EVERYTIME is to establish their Vision as an adult using mind mapping technique if they do not know...ultimately ...then work backwards...do they need to go to college to achieve the vision...what college, what course...
Then backwards again into where they are now and what they need to do.
I then find out what rewards they respond to..money..the spa...travel...friends over etc...often you are not motivated by the reward offered...it has to be something you want!!!
It is true you need to be motivated to do what is in front of you now...but ultimately what keeps a person or even a company growing and moving forward is reaching the goals set that will ultimately achieve the vision,,,,
You will not need everything you learn at school. Lets say you only will use 50% of what you learn in HS, in life itself...the problem is you do not know which 50% so you need to learn everything...
Also set a goal you only want to do each subject once...not hav to repeat it...just get it done!
I hope that help