Welcome to the forum,
@gummi.werm !
Here's my take on school/career progress:
School:
To improve grades:
* Add the "Good School" lot trait to your home lot. This lot trait will make a big difference for better grades.
* Always complete homework and, if available (from B grade), extra credit.
* Go to school in a Focused mood (have them "Ponder Moves" on a chess table around 7:30 am)
(If you have the Spa Day pack, have a parent learn Wellness so they can "Discuss Cognitive methods" with the child before class starts.)
* If they don't bring home a school project, go to Build/Buy mode and buy one for them in the Kids section (click the Violin icon).
* Always choose the "Work Hard" option unless the child loses fun or gets tired/stressed; in that case, switch to Normal (or Make Friends).
* "Make friends" does not improve academic performance at all. I only choose it once their performance bar is maxed with a yellow check mark.
Grade schoolers don't need school friends for good grades.
Career promotions:
* The three most important things are to satisfy the career requirements for promotion, complete the daily task, and leave for work in the proper mood.
You can browse the Careers section of Carl's Guides to learn the best mood for each career.
* If you have the Spa Day pack, put a sim in the right mood by taking a bath soak (with incense), getting an aromatherapy massage, or have a family member do Wellness socials.
(For soaks, massages, and Wellness socials, check Carl's guide on the Spa Day pack.)
For Inspired mood, the sim can take a Thoughtful Shower or cloudgaze.
* Always choose the "Work Hard" option while a sim's at work, unless they get stressed or lose too much fun; in that case, switch temporarily to "Normal" (or "Socialize with Coworkers").
* As with school, I usually only choose the "Socialize with coworkers" option once the sim's next promotion is guaranteed by a maxed performance bar.
The sim doesn't need to befriend ALL their co-workers. Once your sim's met 1-2 coworkers, invite them home (or out on the town) after work and befriend them.
* Quick start: when a sim earns an A in high school, they start their career as a young adult at Level 3.
To start at Level 4, use the sim's satisfaction points to buy the Connections reward trait.
* You can also buy the Entrepreneurial reward trait which can increase how quickly the career performance bar increases.
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So, relationships don't matter at all in school and aren't the most important factor in earning career promotions.
Good luck and happy simming!