Finally finished this achievement just a while ago. Sacrifice a little LP to buy 3-star desks. Get the cheaper 3 LP desk as it is the same as the 5 LP desk. Grind using the Brainstorm (3 minute) achievement while letting your other Sims do other hobbies using the 4-minute activities.
You start with C, then progress to C+, B, B+, A, A+, A++, until you fill the latter to get A+++. It takes 116 or 117 Cram actions to fill A++ level to finally hit A+++. Obviously, I didn't do it normally as that would take a very long time. Here's the secret:
It will be easier if you let Sims to grind hobbies using 4 minute activities while your preteens Brainstorm so that you can earn LP, which will help in getting
Since I had some LP left after doing 1000 GOALS!, I had my preteens do Cram (2 hours). When the timer starts getting below 2 hours, it only costs 1 LP to complete the action.
Don't let the preteens attend school anymore, as they won't progress that much compared to when using the study desks.