Marc Malkin

Lindsay Lohan's gone back to high school.

The starlet celebrated her 22nd birthday last night with a 1980s prom-themed party at Teddy's at the Hollywood Roosevelt Hotel.

The evening started off with a small dinner at nearby Magnolia restaurant...

She wore an off-white slip dress during the meal and then headed into a restroom at the party to change into her second outfit: a pink chiffon minidress by Bradley Bayou. Samantha Ronson wore a tuxedo, including a top hat.

"Lindsay is drinking a Red Bull with a straw," one party guest emailed me during the bash. "There are red, silver, light blue and light pink balloons everywhere."

Lohan's table included an assortment of cupcakes, included an oversize one decorated with a tiara. At about midnight, LiLo was presented with a pink-and-green sheet cake—featuring the iconic Marilyn Monroe photo from The Seven Year Itch—while the 50 or so guests sang "Happy Birthday."

DJ AM provided the music, including Janet Jackson's "That's the Way Love Goes," for most of the night. Then Ronson took over the DJ booth for the last hour until the party ended at about 1 a.m.

Celeb guests included Joel and Benji Madden, David Spade, Audrina Patridge, Sean Stewart and Evan Ross. E! reality stars (and Lohan family members) Dina and Ali were also there, as was Lindsay's brother Michael Jr. and his girlfriend.

Meanwhile, earlier in the day, Lohan and Ronson had lunch at La Scala in Beverly Hills following a shopping trip to nearby XIV Karat jewelry store. Lohan spent $11,000 on a 16-inch briolette platinum diamond necklace and matching bracelet, according to a source.

Ronson is said to have bought a "really beautiful gold bracelet" for the birthday girl.

—Additional reporting by Matt Donnelly