この星でいちばん明るいところ | この星でいちばん明るいところ－ユキの場合－ | Kono Hoshi de Ichiban Akarui Tokoro - Yuki no Baai
From Hoshikuzuu Scans:
Yuki, who’s had his heart broken by his childhood friend runs into a teacher who kisses him out of the blue and tells him, “Let me comfort you”. The perfect teacher with a not-so-honest high-schooler come together in a mature romance.
– chapter 1