Ao no Millennium

12 years ago Chinami was saved in a boat accident and wound up bearing a mark in her hand – the bells that don’t ring. Now she’s in high school and meets a teacher with the same mark. Then someone discovers the mark and tries to separate them – and they suddenly find themselves propelled 450 years into the past!

source: https://www.mangaupdates.com/series/b9seurx/ao-no-millennium