When I was a kid, a company called International Penfriends arranged penfriends. From memory it was based in Finland but had local agents in various countries. Back in the day, of course, there was no internet but it seems these days they have a website.
They do offer a school class service but it looks like individual student details are passed onto multiple potential contacts. They also charge a fee.
I got a penpal from Phillippines because of ipf when I was about 13 years old... Now, I'm 40! We are still great friends in spite of the distance and we talk to each other every day on Facebook :) We never met becase I live in Lisbon, Portugal, and she's still living in Manila.
I've a friend with a 10 year old son who could likely interest the son's class in writing. US in the state of Arizona.
Back when I had a class assigned pen pal (about when I was 10 come to think of it) we wrote the initial letters, put them in envelopes, and wrote on each envelope "to a boy in Mrs. Kaufman's class" or "to a girl in Mrs. Kaufman's class." The class envelopes were put into one large mailing envelope and sent to Mrs. Kaufman by our teacher. Letters after the initial 3 sets back and forth went to our homes, I think.
I, of course, was the smart butt who wrote "to a boy" even through I am a girl. It turned out that my pen pal and I wrote for years, unlike many of the others. Sometimes it is fine if you are a smartie pants. LoL I still recall his name, after 40 years!