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.