I vaguely remember the plot of a yaoi manga and hoping someone can help me with the name of the series!

A student (son of someone powerful) and a teacher. The teacher used to have sex with the principal of the school (might not be sex, but he was definitely doing something with the principal) until the student found out and used his influence to stop it and then he blackmailed the teacher into having sex with him. At one point, the student gave him a blow job while the teacher was teaching and the teacher felt depressed after being abused so often. The student transferred schools suddenly to free the teacher but the teacher missed the student and when he sees the student in a street he confessed his feelings for him.

I think that might be story 6 from Koisuru Omocha.

I think its one of the stories in only you (tohjoh asami) (number six in the description).

I agree with illusion, it has got to be "Indulgent Tyrant" by Tohjoh Asami.

Thank you so much!! It is Indulgent Tyrant by Tohjoh Asami