First, they're asking a highschool student for +450 bucks, how the fuck could he afford such amount of money? The author should've either made him a rich kid or made those gold diggers ask for a sum that an average highschooler could actually afford to pay.
Second, if blackmailing was as easy as taking a slightly embarrassing picture of someone and threatening him to post it online if he didn't pay an absurd amount of money then I'd be a millionaire right about now, I don't thing that the author is even trying to make this manga at least a little grounded, he's just throwing random shit left and right to make the plot move on without paying any attention to logic