Hmm...
By rewards you mean reward traits?
Currently, I don't know any. Though children's aspirations give a reward trait that allows them to raise a certain group of skills faster, reaching to the top of scouting activity gives boost to every skill, and teens with programming skill can hack their grades (which could fail).
But I don't know any reward traits that help with grades. After all, sims only go to school during child and teen life stages. Afterwards, job. Knowing that grades work like job performance and promotions, I think that the reward traits that help with job performance help with grades.
Oh, and Happy Toddler and Top Notch Toddler also boost skill gain.