But first, they had to get to know one another.
Harriton teacher Peter Crooke welcomed the guests and acknowledged that both groups were a little out of their comfort zones.
"This is a leap for us to have people come in from a neighborhood we're not familiar with. And for you, too, to come out to a neighborhood you're not familiar with," he said, motioning to the Philadelphia students.
But they got quickly down to business - an icebreaker, a tour of the school, a $100 million structure opened in 2009.
Harriton junior Carlie Ladda pointed out the cafeteria to Erica Irving, a senior from Bodine High, a magnet school in Northern Liberties.
"It's pretty good," Ladda said. "They have a sandwich bar - it's like a Wawa. They made whatever you want."
"This is like four times the size of our lunchroom, and that doubles as a gym," she said.
But the students found a lot of common ground, too - the sports they play, the clubs they participate in, colleges, dress codes.
Harriton junior Ilissa Kaye pointed out the theaterlike auditorium, the greenhouse, the lecture hall.
"Can you carry your bags in class here?" asked Aliyah Pressley, a junior at Mastery Charter School's Lenfest campus.
Kaye said they could.
"We can't," said Pressley. "You can't even get into the school with your jacket on."
The Philadelphia students said they liked the way teachers seemed to treat students with respect at Harriton.
But not everything was better at Harriton. TiJuana Jackson, another junior at Bodine, said that the Lower Merion school felt too big, with 1,000 students.
"I like our school because we know everybody," said Jackson. About 550 students attend Bodine.
Later, back in their meeting room, the students told Crooke they thought they had more similarities than differences. But the differences were eye-opening.
The Philadelphia students said they liked Harriton's library - some of them have no libraries at their schools - and the fact that every student gets his or her own laptop. They liked all the languages, all the AP courses, all the activities and creative outlets offered at Harriton.
It was a marked contrast to their schools, they said.
"We have to bring our own paper to one class," said Jackson.
Kaye looked aghast.
"You should start a petition that all ID fees go to copier paper," she said.
Why are things so different in Lower Merion, Crooke asked them?
"Taxpayer dollars?" said Harriton junior Cat Dolan. "Here, a lot of parents put a lot of emphasis and money into schools."
Crooke pointed out that although Lower Merion has a relatively low tax rate, the incomes in the town are high enough to support an excellent school system. Plus, he said, it doesn't have to pay for all of the infrastructure and service costs a city like Philadelphia does.
(Philadelphia's state funding also got cut more deeply than any other district's in the last budget cycle.)
Dolan pointed out that students everywhere tend to feel stifled. At her school, "a lot of us feel like school is jail - we walk in circles, some of the doors lock automatically, and there are cameras everywhere." But in Philadelphia, many schools have bars on the windows, metal detectors at the entrances, and police officers in the hallways.
"It's a different perception of what jail is," Dolan said.
Lewis spoke up then. On his way to meet the UCCP staff to go to Harriton, he was stopped by Philadelphia police, who threatened to haul him in to a truancy center.
"In Philadelphia, if they see you on the street, they can pick you up and put you in the back of a police car like you're a criminal," said Lynsey Graeff, a UCCP staffer and Temple student.
The room was quiet for a minute.
"Takes the joy out of going on a field trip," Crooke said.
The conversation went on for more than an hour, spilling into lunch. When it was time for the Philadelphia students to return home, there was no doubt - everyone wanted to do more.
The next time, the Harriton students will visit Philadelphia. Down the road, a formal partnership is possible.
"We're seeing there's a bigger world, and that we can do real work," said Crooke.
Contact staff writer Kristen Graham
at 215-854-5146, email@example.com, or on Twitter @newskag. Read her blog, "Philly School Files," at www.philly.com/schoolfiles.