(ABC News) -- High school graduation and prom season have arrived, and along with the corsages, caps and gowns come the parties. It's a notion that can strike fear into the heart of any parent, and for good reason. These days a child's bad behavior doesn't just lead to disappointment but can actually lead to a jail cell -- for the parents.
Ask Paula and Barry Spencer, from Falmouth, Maine.
Last June, after their 18-year-old son Nicky's high school baseball team won the state championship, they agreed to host a celebration for the players. The Spencers set out the ground rules: no more than 50 guests, and absolutely no drinking or drugs.
As guests started arriving around 7 p.m., they were served barbecue along with lemonade, iced tea and water. In an interview with "20/20," the Spencers said they weren't naive to the fact that their teenage guests might try to sneak in something harder.
"I thought people were going to attempt to bring in beer and alcohol, but we were watching for it. I was actually right at the front of the house watching kids come in," Barry Spencer said.
As the sun set, the atmosphere at the party started to change. More kids were arriving, and Paula Spencer, who was stationed at a window looking out onto her backyard, had a harder time monitoring the scene.
At 10:30 p.m., an anonymous tip came in to the Falmouth Police Department that the Spencers were hosting an underage drinking party. Sgt. George Savidge responded to the call.
"I pulled up to the house and there were probably 30 cars in the roadway. Immediately a gentleman came out and identified himself as Mr. Spencer. He said he was hosting an alcohol- and chemical-free party. He asked that I help them monitor the area for kids that would sneak in with alcohol or drugs," Savidge said.
Savidge told Barry Spencer he would monitor the street, but that Spencer was responsible for his property. Savidge saw no evidence of underage drinking, so he left.
But by 11 p.m. the party had ballooned to around 100 rowdy teens, some of whom had snuck in alcohol and many of whom were now openly drinking. Rather than shut down the party, the Spencers attempted damage control, dumping whatever alcohol they could grab down the sink.
"I invited these kids to our home to celebrate, and I didn't have clear evidence that they were all drinking. I just felt like, is it fair to shut it all down because of a few kids spoiling it for everyone else?" Paula Spencer said.
At 11 p.m. another Falmouth police officer made a traffic stop and found a drunk underage passenger in the car. When the officer questioned her, she admitted that she had just come from the Spencers' party. Police decided to head back to the house.
"At this point everyone seemed to be in a much higher state of intoxication. Now it looked like there was a party going on," Savidge said.
Police reports from that night noted 20 to 30 alcohol containers scattered around the back patio, a large marijuana pipe sitting on a table, vomit on the ground and a teenager passed out on a neighbor's lawn.
The police Breathalyzer-tested underage partygoers who were trying to leave, and before the night was over the Spencers had been summoned for furnishing a place for minors to consume alcohol. Police noted that they could have arrested the Spencers on the spot but agreed that since many of the teenagers were sleeping over it would be detrimental to remove them from the home.
Laws targeting parents who knowingly or unknowingly allow drinking to occur in their homes have popped up all over the country. Furnishing alcohol to minors is illegal in all 50 states, but it can be difficult to prove who actually bought the booze. In response to this challenge, 28 states have adopted "social hosting" laws, which impose criminal penalties on the host of a party where underage drinking occurs. That number is up from 18 states in 2005.
The Spencers went to trial last February, and Cumberland County District Attorney Stephanie Anderson was determined to have them serve time.
"The people responsible for making sure that drinking didn't go on at that party were Paula and Barry Spencer, and they did not take appropriate steps to prevent it," Anderson says.
But Anderson could not get the whole jury to agree with her. The case ended in a mistrial, with the jury split right down the middle. Rather than retry the Spencers, Anderson offered them a deal. They would not serve jail time but would pay $17,000 in fines and restitution, write a letter of apology for the local newspaper and serve 100 hours of community service each. The Spencers took the deal.
"We did not intend for kids to drink. It happened, and I'm sorry that it happened," Paula Spencer said. Although the reaction from the community and local media has been harsh over the past year, the Spencers decided to tell their story so that other parents could learn from their mistake.
As for having another party, the Spencers said that next time, they would ask a parent or two to stick around.
Watch the full story on "20/20: With Parents Like These" TONIGHT at 10 p.m. ET.
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