High school students at more than 18,000 schools are preparing for May’s Advanced Placement exams, which bestow college credit upon high scorers, Mashable reports. Now Macmillan’s adaptive learning startup is preparing to market its study tools directly to those students for the first time. The company Prep-U [1] is launching AP test-prep apps for US History [2], AP Chemistry [3] and AP Psychology [4] on Thursday. Based on technology previously sold as an online study companion to textbooks, the apps first determine a student’s current mastery level through a series of quizzes. Thereafter, they serve up quiz questions at that level. The idea is to never ask a question the student already knows for sure or one that will completely stump them, thereby focusing attention where it’s needed…