“You can’t sit with us” isn’t just a “Mean Girls” quote … it’s apparently a very real rule in Congress … and Rep. Sara Jacobs says she’s had to use it IRL!

TMZ caught up with the Democratic California congresswoman in Washington, D.C. on Thursday, and she spilled the tea on Capitol Hill being basically one giant high school … with cliques and cool kids and unofficial rules.

Watch the video … she takes us on the underground Congressional subway train and breaks down how the House floor’s lack of assigned seating doesn’t mean you can just plop down anywhere you like … because groups like “Penn Corner” and “California Row” already have their unofficial spots locked down like a school cafeteria.