Hi :)
Well, I find fun is easily raised by having Sims view Decorative items- statues, paintings. There are career rewards that boost needs- such as the Adventure Career, I think the Journalism career, and maybe the Entertainment, too.
Apple juice is supposed to make them finish homework faster; strawberry lemonade puts them in Platinum...
Yes, having an older family member help with homework does make it go faster. A teen can help a child and an adult for a teen (or elder). :)
An hour of homework at night, play or view decorative items, let them have fun or skill on fun items like chess and paintings then in the mornings they can finish their homework.
They'll eat at school if they go to school hungry- not too hungry. :)
If any Sim can earn the Gold flower arranging badge to make Snapdragons then those will help all needs but energy.