Schools across the U.S. are upgrading doors, locks and other protection systems as they look to keep students safe from shooters, driving sales for security-product manufacturers.

Companies including Honeywell International Inc., Allegion PLC, 3M Co. and others are selling cameras, alarm systems, fortified doors and security films, which make it harder to crash through glass windows or doors. Manufacturers are developing new products as well as adapting some they already sell, industry officials said.