Deborah Grubbs, an elementary school teacher, began concealing Dr Pepper, her favourite beverage, in a coffee cup last year, when her private school in Texas banned soft drinks. When her birthday came round, her students needed special permission from the headteacher to bring her the treat to celebrate.

She felt as if she'd been "convicted" of a crime, Mrs Grubbs said. "There's a difference between adults and kids. That just seems over the top to me."