Both have a high school setting with a strong lead male character who like their homeroom teacher. B Reaction is more focused on fighting whereas it's not very regular in UnbalanceX2. Although not quite a harem there is more than one girl who is after the male protagonist. B reaction is a bit short but definitely worth reading if you liked the excellent UnbalanceX2.
Both manga\manhwa revolve around a rather strong highschool student who has a crush on a (female) teacher at his school. Oddly enough, in both series it's highly indicated that it's actually not a one side thing. Unbalance x Unbalance is a bit more serious (and with lots of drama) than the other series, but there are only a few action scenes. On the other hand B.Reaction! is filled with action and fights. Not to mention, not only is the teacher strong, but she is actually stronger than protagonist. If you liked one of those two series (or the f.teacher x m.student theme) then you should definitely give the other one a try.
What could possibly 3 unrelated series like Unbalance x Unbalance, Enchanter and Gokusen have in common?
In all 3 of those series, the male lead happens to fall in love with a female teacher and in all 3 it takes him a lot of work and maturing to make her feel the same way about him as he feels about her.
Yamaguchi Kumiko is next in line to be the boss of the Kuroda Ikka yakuza clan due to her parents' death. However, fresh out of college, Kumiko decides to pursue her lifelong dream of being a teacher. Now hired as a mathematics teacher at an all-boys school, Kumiko must hide her true identity. She's assigned to class 3D, inhabited by the worst of the worst delinquents. Popularly called "Yankumi" by her students, she tries her best to gain the respect of and offer help to these students who'd like nothing more to make her life a living hell.
Male leads in both Unbalance x 2 and Gokusen happen to fall in love with their homeroom/math teacher (a female one in case you wonder). They are both quite serious about it, but it takes them a lot of maturing to achieve their goal which is, make their object of affection feel the same way about them. Unbalance x 2 is mostly driven by drama (lots of drama) and romance, with some comedy on the side, on the other hand Gokusen is mostly driven by comedy and fights, though the romance is still there. Both of those series resolve the romance part pretty well, so if you liked one of them because of the romance, then you'll probably love the other one too. Just keep in mind that Unbalance x 2 had those bits of ecchi and mature undertones (occasionally).
Onegai Teacher is another series with student x teacher romance. It's lighter on drama and there is this silly UFO plot, but it still is similar to Unbalance x 2. If you liked Unbalance x 2, you might want to check Onegai Teacher too.