Shintaisou: Kari

Similar art and animation style, noticeable in characters facial expressions and body movement, as well as theme (schoolgirls getting raped). 

extreme hardcore no-mercy ''rape''. Similar artwork, similar direction, similar sex scenes (toys, bondage, enemas, shocked girls etc). dark love has a bit more gentle but still rough storyline. 

Same Character Designer, similar animation, same director and the combo "Teacher + Rape". If you liked one, you may like the other. 

They both involve the students and the teacher, they have similar settings, the characters are similar, both story lines are similar.The teacher character in both of these are very similar, none of the students like him, but there is one student who doesn't mind him, and so the Teacher manipulates the students, and use and abuse them. if you liked one, you will like the other. 

It's all about hardcore schoolgirls rape. Similar artwork, similar direction, similar sex scenes (toys, bondage, enemas, shocked girls etc). Gakuen 2 has more dramatic storyline.