@PROXIMITY
I second the Gal Gohan recommendation. It's really sweet and silly. There's some mild harem stuff but it's never that serious. It's not so much a harem as it is that there's small amounts of competition for the fMC, but not in a way that ever really gets in the way of the main couple.
And just regarding the others, Shiota-Sensei to Amai-Chan hasn't updated in like 7 months (though the translation group leader is still active on the site, the group's page has no new releases since its most recent chapter of that title)
meanwhile Tsuiraku JK to Haijin Kyoushi might be a little off-putting as there are some mild dark elements to it (just read the description to see what I mean.)
Beyond that, one of the perspective couples (Suzu & Hikari) of Zense Coupling is a teacher-student pair. And really that whole manga is fluffy as hell so it's a fun read. If you really wanted to you could just read the Suzu/Hikari marked chapters for just their story and ignore the others, but really it's all good.
Sensei x Seito Anthology includes, I think, the basis of Shiota-Sensei to Amai-Chan along with a bunch of other teacher/student one-shots. I haven't read all of it yet though.
Kou 2 ni Time Leaped Shita Ore ga, Touji suki Datta Sensei ni Kokutta Kekka is a bit of a cheat (the mMC time travels back to his high school days to confess to the teacher he liked but couldn't work up the courage to say anything to. So he's technically not a high school student but honestly like 90% of the time they don't even bother remembering the time travel/he's actually older part of the story) but it's still enjoyable enough. EDIT: and I see someone else recommended it while I was writing this.