Responses
http://www.mangago.me/read-manga/naisho_no_jikan/
maybe this manga but I don't remember if the boy is blackmailing her.
Omg, this is it! Thank you soooo much!!!
Anyone knows a manga which the boy and the girl were in the same school(maybe in the same class). The girl was working as a maid in a coffee for mature men, she had a poor family that she had to support. Working was banned by their school so the boy was blackmailing her when he found out about it. Once he came to the coffee as a customer. The manga was not too long. Also it wasn't the Kaichou Wa Maid-sama. Thanks in advance :)