'Laguna Beach' Star Lauren Conrad Celebrates 40th Birthday with HS-Themed Bash

Lauren Conrad turned the big 4-0 earlier this month -- but her birthday bash brought her all the way back to her high school days on 'Laguna Beach.'

The former star of the popular reality show enjoyed a high school-themed 40th bday bash, which captured the spirit of her actual high school prom that was filmed for one of the "Laguna Beach: The Real Orange County" episodes from 2004.

On Friday, Lauren posted a carousel of Instagram photos from her "Senior, Senior Prom" soiree and, as you can see, she's beaming from ear to ear with all of her friends and her musician husband William Tell.

At one point, Tell hopped on stage with his electric guitar and cranked out some tunes for the crowd.

In the caption, Conrad wrote that she celebrated her 40th with 3 of her favorite humans, namely Tell and 2 friends, all of whom were born a few weeks apart.

Conrad noted they all decided to throw themselves a prom to celebrate together -- and "had the BEST time!”

Some of the 'Laguna Beach' cast gave shout-outs to Lauren in the comments section, including Morgan Smith and Talan Torriero. Unclear if they made it to the throwback shindig.