The students were on the last day of a tour of Mayan ruins. Guatemalan police later detained two men.
The crime, last Friday, seemed certain to hurt tourism in Guatemala, favoured by backpackers attracted by its Mayan culture.
The Foreign Office warns travellers to beware. "Violent crime is common throughout Guatemala," it says. "Muggers are often well armed. Visitors should be alert at all times, and not travel alone, especially at night. Armed attacks on long-distance and city buses are common."