Her senior year was just a month old at Newfield High School in Selden when Tiana Rapp, 17, opened her gym locker and discovered someone had pilfered a whopping $150 from it.

Police have arrested a hall monitor, Linda Cubano, and charged her with three counts of petty larceny in connection with the thefts.

Rapp said she had reported the missing money to school officials, who suspected a student was responsible. But something didn't add up, she said, because the girls' lockers were patrolled by an adult hall monitor.

The criminal struck again a couple of weeks later: That time, Rapp was left $40 poorer.

"They said there was nothing they could do and they were convinced that a student was doing it," said Rapp, a fan of the crime-fighting television series "CSI," and an aspiring forensic scientist who has already aced the SAT and her Advanced Placement psychology class.

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Middle Country school district officials released a statement Thursday night declining to comment on the case, but they did acknowledge that they had accepted Cubano's resignation.

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Initially, Rapp didn't buy the adults' explanation of the thefts, but she did know that her losses had begun to add up.

That's when Rapp and a friend with whom she shared a locker, a girl who also lost $20 in the second theft, devised a plan to catch the thief.

On Monday, the teen sleuths set up a video camera inside a locker, propped it up just so and trained it on the locker containing their belongings.

Sure enough, Rapp said, 12 minutes into the video, it appears that the hall monitor walks by the locker containing Rapp's silver handbag, opens the locker with a master key and begins rifling through it.

"She was rummaging through everything," Rapp said.

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The girls showed the video to school officials and a Suffolk County police detective - who later showed the footage to Cubano, 54, of Selden.

On Tuesday, Cubano was charged with three counts of petty larceny after she admitted swiping students' belongings, police said. She was issued a desk ticket. Cubano could not be reached for comment.

Rapp hasn't gotten her money back, but she said she's happy she snagged a suspect on the first try.

"I knew there was no way a kid was breaking into the locker without anyone noticing," Rapp said. She added that she's been thanked by several other students whose belongings vanished from their lockers.

Tiana's mother, Kelly Rapp, said her daughter served on a school safety committee last year and that she had raised the issue of having items stolen from her locker at that time.

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"I think it's great," Kelly Rapp said of her daughter's investigative work.

"Because someone had to do something, otherwise it would just continue. I'm very proud of them for doing what they did."

>>PHOTOS: Click here to see photos of the teen, and hall monitor