The nation's second-largest teacher's union has ended a promotional program with Wells Fargo, citing the bank's continued ties to the gun industry.

The American Federation of Teachers, representing 1.7 million workers, said it removed the Wells Fargo mortgage program from its member benefits site effective Thursday. The union had attempted to discuss its issues with Wells Fargo management but didn't get a response, it said in a letter Thursday that was reviewed by CNBC.

Wells is a lender to gun-makers and the National Rifle Association.

The union is part of a national wave of activism trying to get the firearms industry to improve gun safety and sales policies after another deadly mass shooting, this time at a Florida high school in February that left 17 people dead.

Major car-rental companies, hotels, airlines and others have ended rewards and promotional programs with the NRA in recent months. And financial institutions such as Citigroup and Wells Fargo have limited or curtailed business with gun-makers, though Wells Fargo has not done so.

"Despite our several attempts, by phone and email, to schedule such a meeting, your office's response has been radio silence," said the letter, signed by the union's president, Randi Weingarten. "We can only assume," the letter added, "that the NRA business is more valuable to you than students and their educators are."

About 20,000 AFT union members have mortgages through the program.

In an interview in March in the Charlotte Observer, Wells CEO Tim Sloan said, "I don't know if banks or credit card companies or any other financial institution should be the arbiter of what an American can buy."

In a statement emailed to CNBC, a Wells Fargo spokesman said, "We remain deeply committed to the financial success of teachers and all of our customers. Wells Fargo wants schools and communities to be safe from gun violence, but changes to laws and regulations should be determined through a legislative process that gives the American public an opportunity to participate. We remain firm in our belief that the American public does not want banks to decide which legal products consumers can and cannot buy. "

The teachers union is also putting pressure on its pension managers, who oversee $3 trillion of teacher retirement savings, to push fund companies to shed gun-maker stocks, offer funds that specifically exclude gun-related investments or drop investment managers that refuse.

Correction: An earlier version of this story incorrectly said the American Federation of Teachers is the nation's largest teacher's union.