Ano Kado Wo Magatta Tokoro
Two people who meet by chance at a streetcorner one night fall into a casual relationship, but when one takes a job student teaching, he realizes his partner - who had claimed to be a 19 year-old college student - is actually a 16 year-old in his second year of high school. **Spoilers** This is an interesting story with a solid relationship, but I wish Kaga had better explained to Yuuya why, legally, continuing their relationship once he'd found out Yuuya's age was a bad idea. Say what you will about Yuuya's maturity level and ability to consent in this scenario - the school is right that legally, he's a minor. It wouldn't have been the same story without that drama built in, but if they were both willing to wait until after graduation, they could've done that from the start without the forced separation. Still a challenging situation to consider and a story I've happily re-read.
Devils Honey
The person who changed a young man's life years ago comes back into his life as the phys ed teacher at his high school.
Yukidoke no Koi
The teacher/student grow close but kept things professional (snow day aside), have shared interests, the student makes all advances, the teacher puts a stop to them, and only after graduation do they discuss a relationship or do anything physical. They genuinely cared for each other and I really loved their story. I especially liked that our MC came out to his best friends, who were lovely and supportive. A+ for that. His friends have their own story (and these two have a cameo!) in the manga Bokura ga Koi wo Ushinau Riyuu. I tagged both with this story's title so they're easy to find. The seventh chapter is a story of unrequited love and being someone stand-in. It's bittersweet but not hopeless. Stings, though. The second-to-last chapter is about two men who never told each other how they felt while they were at school and met twelve years later, when one is married and the other thinking about marriage. They have to hurt the people in their lives or recognize this goes no further. The final line is about a fruitless tree making fruit, so it rings hopeful. The final chapter is our MC & his sensei when the MC is away at school, on a weekend visit. :-)
Young Bad Education
One evening, a high school student finds his homeroom teacher drunk on a bench and is shocked when the teacher kisses him, confessing he's watched him since his first year but has tried to forget him. Angry but interest piqued, the student proceeds to stalk the teacher, gradually eroding his self control. Teacher student with a teacher who is actually conflicted about the way he feels!
Sensei! Koi No Shitsumon Desu!