Kristin Cavallari Says Her 'Bad Girl Era' Began in 4th Grade

Kristin Cavallari is kicking off the new school year by detailing her "bad girl era" ... and she says it all started when she was only in 4th grade.

Check it out ... on today's episode of Cavallari's "Let's Be Honest Podcast," she told her millions of listeners her 4th-grade teacher read through her diary, which had the word f*** written in it.

The reality star disclosed her 8th grade school year later became the "turning point" for her rebellious ways ... faking sickness and calling out, piercing her bellybutton with safety pins, taking trains without an adult, and quaffing airplane bottles of booze at school.

K-Cav also had her fair share of run-ins with the cops, dating back to middle school ... and continuing on to her days as the popular girl at Laguna Beach High School in Southern California.

She recounted when she and former boyfriend Stephen Colletti pulled over while having an argument ... cops came and escorted Kristin -- who was not yet 18 years old -- back to her father's house.

Another pastime of the "too cool for school" gal? Shoplifting -- which came to a halt after getting caught and receiving 100 hours of community service. This was the summer prior to landing on MTV's "Laguna Beach."