Leta wanted what she could never have. Newt. Newt as in not only her friend but her soulmate. To bad that Newt has been soulbonded since birth, indicating that his partner is older than him. The incident at school was her attempt at destroying Newt's soulbond with Graves and taking his place.

The attempt failed and she covered up her tracks. Newt's soulbond was damaged so badly that Graves believes his soulmate to have died which causes him to become cold and aloof.

Fast forward to the end of the movie when they meet for the first time and to the suprise of everyone the souldbond starts to heal.

+ the type of soulbond is up to the author (first sentence, tattoo, color blindness or something else)