Not much can be said about this manga - simple 'no brainer' with some 'Plot'. It's all about an ordinary boy, a teacher, and some girls (including sister and little girl from elementary school). All of them are in love with MC. Reason? There's no reason, or it's pretty absurd. Actual plot is straightforward and predictable. It only got couple of chapters so far, so it's hard to say if MC will try to build harem, or how the character developement will go. So far it's all about the MC, the teacher and their relationship.

I will try to be as objective as possible. In every aspect the starting value is 5, which is mediocre:
Story: 2/3
As mentioned before, it's nothing special. Chronological storyline with classic flashbacks. Predictable, straightforward, or even boring one would say (there's a lot of MC monologues that doesn't really add anything interesting to the story).
Art: 6
Art is looking pretty fine. It's nice to look at, there's a lot of details. The main problem is that everything looks alike. All characters look very similar, both in terms of overall appearance, but also in terms of clothing and facial expressions. It's not particulally bad - just nothing special, as usual.
Character: 3/None
It's hard to say how the character developement will go. So far (I'm around chapter 11 writing this review) we only know some things about MC, his sister and teacher back story. And so far, it is nothing memorable or revealing.
Enjoyment: 3/4
It's simple manga. Do not expect some deep storyline or amazing charackter developement. There's plenty of ecchi stuff (all censored by default) if you are looking for that.
Overall: 4
Of course there are many better romance/ecchi type mangas. Boku no Kanojo-sensei is nothing special, nor amazing. If you are looking for a engaging storyline - it's not here. But if you want something which doesn't require much attention, simple 'no brainer' manga for chilling between work/school, this might do the work.