This achievement requires that you have 3 preteen Sims with a grade average of A+++. To obtain this achievement, you must first have three preteen Sims. A preteen Sim evoles from an infant Sim. Once you are married, buy the crib and then have a baby (costs LP). Bake a birthday cake (costs LP) and use it on your infant to age them to a toddler. Then bake another birthday cake and use it on the toddler to get a preteen. The grade averages progress as follows: C, C+, B, B+, A, A+, A++, and A+++. You raise a preteens grade average by:
Send them to school - Build the Sim Town Elementary. Preteen Sims can attend the school during the daytime, similar to they way adults go to work. Each time they go to school, they will earn XP towards this achievement. However, it is a very small amount.
Use a 3-star study desk - Purchase a 3-star study desk from the store. This is not available for purchase until Level 17. It costs 3 LP. Using this (for any length of time) is the fastest way to increase a preteen's grade average.
You MUST have all three at this rating at the same time. A preteen's grade will drop slowly in time if they don't study and go to school. Make sure you have three preteens with an active A+++ grade average to pop this achievement.