Grace Etchells, a former classmate of Alex in Hadfield, Derbyshire, said: "I knew Alex from primary school - St Andrew's Church Of England Junior School - up until about six years ago.
"That is when Alex, Hannah (her younger sister) and her mum and dad left for the USA.
"We were classmates for about three years.
"She was a great girl - she had zero flaws."
Ms Etchells also took to Twitter to pay tribute to the teenager.
More top news
Iceland v Iceland: The Nordic nation presses on with legal action after talks with British food company fails.
Not as cold as recent nights - any frost will be confined to parts of Scotland and East Anglia where temperatures will dip below freezing.
A police chief constable has defended his force's handling of child sex allegations against former prime minister Sir Edward Heath.