Sekai wa Bokura no Tameni!

From Midnight Scans:

Starting this spring, citizens will be able to become a teacher at the expensive high school! This is what Maho is getting excited about, but her boyfriend for six years dumped her. Afterwards, she slept with a young boy to get over her sadness. However, who would have expected that she would be reunited with him in the classroom. That's right, he was her student!! A secret relationship with a younger handsome boy is packed full of scary, yet, exciting, nerve-racking, yet, heart-thumping action?! What will happen when the man who dumped her comes back?