I noticed that as well with some Sims already starting out with a boarding school as their alma matter. It may be related to the Sim's traits according to each school's learnable traits.
Smuggsworth Prep School
Positive Traits
-Ambitious
-Charismatic
-Snob
===
Negative Traits
-Couch Potato
-Easily Impressed
-Inappropriate
-Rebellious
-Slob
Fort Starch Military School
Positive Traits
-Brave
-Disciplined
-Good
-Handy
-Neat
===
Negative Traits
-Clumsy
-Couch Potato
-Inappropriate
-Insane
-Loser
-Over Emotional
-Party Animal
-Rebellious
LeFromage Art School
Positive Traits
-Artistic
-Charismatic
-Dramatic
-Photographer's Eye
-Savvy Sculptor
-Star Quality
-Virtuoso
===
Negative Traits
-Can't Stand Art
-Couch Potato
School of Peace and Love
Positive Traits
-Artistic
-Eco Friendly
-Green Thumb
-Loves the Outdoors
-Vegetarian
===
Negative Traits
-Hates the Outdoors
-Mean Spirited
-Snob
Dribbledine Sports Academy
Positive Traits
-Adventurous
-Athletic
-Disciplined
-Handy
-Loves the Outdoors
===
Negative Traits
-Clumsy
-Couch Potato
-Hates the Outdoors
-Loser
-Rebellious
-Slob
