52 Hertz no Kyoushin

Shirane is a high school teacher and omega who tries to lead a regular life. When he is put in charge of a school trip, he meets his high school classmate Kiyonari again at a meeting with a travel agent. While he feels nostalgic, he is affected by Kiyonari, who is an Alpha and is suddenly attacked by heat. When Kiyonari finds out that Shirane is an Omega, he offers him a proposal to take care of his condition......?

(Source: Homesha, translated)

Note: Includes one extra chapter.