In this manga, a student flirts with her teacher, and the teacher has sex with her, impregnating her. From there, she decides to drop out, has the baby, the teacher quits or is fired, and they get married. All of this happens before the manga or right at the beginning. Most of the manga depicts their life after getting married, the difficulties they face, etc. I think this manga is pretty short (about 5-10 chapters), though maybe I'm just misremembering. It may be more than that (maximum 30).

Ichigo no Gakkou

That's it. Thanks!