The thing up his ass could've been an vibratory or a buttplug, either way it was very easy to see even through his pants like it's poking out. Also I think the seme was another teacher or a student or something I can't remember
Base on this description i thought it's Quickie by Zenra QQ, all your description points to it but, in Quickie there's no seme tho. The s*x scene is all the uke's imagination if i'm not mistaken.
OMGG THANK UUU!!!!
You're Welcomeee!!! (=・ω・=)
Looking for yaoi about a school where every student is a pervert except for one teacher, every students have sex even during class but the teacher doesn't care. But actually the teacher always has a dildo up his ass and the students know it. At the end the teacher is teaching class but the students noticed he is not wearing the dildo anymore so they're thinking maybe he now has a boyfriend.