Katy Perry may be 27, but that doesn't mean she can't hang with teen superstars like Selena Gomez and Justin Bieber -- and perhaps act like a middle schooler. The starlet had a grand time at her 'Part of Me' premiere, despite being photographed with tissues stuffed down her dress!

Following a series of outfit and costume changes throughout her big night, Perry made her exit from the premiere in a party bus with Gomez and Biebs. She rocked a black corset-style dress with giant navy blue polka dots, complementing her cool, purple-toned locks. The lowcut gown showcased -- as they so often do -- Perry's ample cleavage (and looked rather uncomfortable -- girl, it's okay to go up a size and get your ish tailored!). And tucked within that cleavage? Tissues.

Since it's been well-documented (and often most vocally by Perry herself) that Perry's well-endowed up top, her Kleenex likely weren't to boost her cup-size, but maybe she was sneezy or sniffly. But still... she has assistants and probably a purse. Does she really need snot on her boobs?

In any case, she had quite the crew on her party bus. Aside from Jelena, Perry was joined by none other than 'Twilight' star Robert Pattinson, who was almost accidentally shut out of their fun ride. Watch him try to hop on in the video below! (And no, Twihards, his lady love Kristen Stewart wasn't present -- but he and K.P. are just pals, as she hung out with the couple at Coachella!)

Watch Robert Pattinson Try to Get on Katy Perry's Bus