Marisa Henrie sorted through a pile of stamps, placing them in diverse categories like swimming and mushrooms.

“I’m trying to find a good topic,” she said. “My friend likes Polska stamps, so I’ve been setting them aside, and my sister likes space stamps.”

Marisa, who has come to find sorting through stamps “very addicting,” settled on a theme of hot-air balloons for her stamp display.

Stamp-collecting was a common hobby decades ago, but she was not collecting stamps in the height of its popularity. She is a sixth-grader and was sitting in a room with 11 other Cutler Middle School students. Through Robert Welt, the hobby of stamp-collecting lives on in another generation.

Welt, 70, is a retired history teacher in Groton Public Schools but returns to Cutler every Thursday after school to lead the stamp club, which is roughly 60 percent concerned with stamps and 40 percent with old letters.

Welt taught at Cutler and then spent 28 years teaching at Fitch Middle School. He started the stamp club in the early 1990s, and when Fitch closed in 2012 and he went back to Cutler, the stamp club came with him.

Welt said after he retired in 2014, a French teacher picked up the club for a year but implored Welt to come back. So he did.

Every year Mystic Stamp Company – not based in Mystic – sends Welt its stamp catalog and some beginner kits. And every year Welt takes students to the Philatelic Show, held in Boxborough at the Northeastern Federation of Stamp Clubs.

Eighth-grader Alayna Woodhams says they call it “Nerdfest.”

When Welt does exercises asking students to identify the scene going on in a stamp, he calls them “white socks exercises,” because “stamp collectors by definition are nerds, and nerds by definition have a pocket protector full of pens, wear white socks with a business suit, and fix their glasses with missing tape.”

In a closet at Cutler that used to be a telephone booth, Welt keeps two large bins full of stamps and other supplies. Some of them are donated, and some he buys in bulk.

The club has also gotten financial donations from the American Philatelic Society and the Northeastern Federation of Stamps.

Sitting in different groups at one Thursday meeting, students worked on different projects. Some sorted stamps while others transcribed letters.

Looking at a letter from Dec. 29, 1926, Kate Tarinelli and Rachel Carney couldn’t get over how sad it was.

“Now he’s broke, and now he says everything is dull, and he has bills to pay,” Kate commented. Rachel later called Welt over for help with a word that was particularly difficult to decipher, in its cursive scribble.

Sometimes kids work on the same project for a year or more, scanning difficult-to-read letters from centuries past to identify the various members of a family. Seventh-grade student Noah Johnson has been working on a project on the Rev. Birdsey Noble since he was in fifth grade.

Sixth-grader Chandru Garlapati has completed several displays of stamps on colorful sheets of paper, with themes like birds, the Olympics, and the Republic of Malgache, or Madagascar.

Students often join the club after hearing about it from a family member or friend – like Elaina Brookhart, whose sister was in the club.

“It’s just an interesting experience and it’s really just fun,” Elaina said. “It’s the highlight of my Thursdays.”

e.moser@theday.com