THE UNDYING ENGAGEMENT SEAL OF THE DOOMED WITCH AND THE YOUNG MASTER WHO LEAPT THROUGH TIME

The Undying Engagement Seal of the Doomed Witch and The Young Master Who Leapt Through Time summary:

Michela is a 13-year-old student at the Clausura Academy of Magic and Sorcery, a school for magic-holders aspiring to become “Wizards”. Unfortunately, she lacks even the most basic control over her powers, with all her attempts at spell-casting inadvertently ending up in explosions.
One day, Michela sets off a particularly nasty explosion that stood to consume anything and everything around her.
Just when death seemed imminent—
A young man appears before Michela, deflecting the blast in the nick of time.
Introducing himself as Eusius, the young man tells of a foreboding tale: that he has come from the future to save her... and that she’s fated to die at the young age of 18......