Kojin Jugyou

(from shoujo-manga.com)
Michiru is a troubled high school student. She has just told a huge lie to all her friends, and she is about to be found out. They all think she has this fabulous boyfriend, but it isn’t true. And now they all want to meet him, and Michiru has no one to introduce. In the day of the big gathering with her friends, where she is supposed to show off her fabulous boyfriend, Michiru stands alone, in dread, waiting for her friends to realize she lied (they already are suspicious of her claim). But things take a sudden twist when Michiru’s “boyfriend” actually shows up and sweeps her off her feet! No one is more shocked than Michiru to see this handsome guy acting as her boyfriend. What is even more shocking is who he really is!
This book contains 2 other stories. One is yet another student-teacher story. The other is a romance between two high school students who have known each other forever. He has some blackmail material on her, and uses it to basically make her his slave. Real feelings emerge when she finally resists being his toy.