Afterschool activities are really useful for your student Sims as they help them excel in skills depending on the activity they are partaking in. For example: Scouting develops Fishing, Art develops Painting, Music develops instruments and so on and so forth. They also get little interactions called 'Show Off' which they can perform to other sims as they are learning.

It takes two hours out of selected afternoons but the little interactions they learn and the ceremony and reward they receive at the end are worth it.

Note: when your Sim becomes a teenager they have much more of a variety of Afterschool Activities: Drama, Sports, Debate, Art, Music, Newspaper, Shop and Study.

Drama = Charisma
Sports = Athetic
Debate = Logic
Art = Painting and Scultping
Music = Guitar (will also raise Bass, Drums and Piano if you have Late Night)
Newspaper = Writing and Photography
Shop = Handiness
Study = Homework

A scout showing off his salute.