I chose "Study" every day and got the Normal Bad Ending, then tried again managing to get the Scientist Ending using Rosalie's advice

You can not simply choose to "Study" every day and expect to get the "Scientist Ending" if you study every day you will get the Normal Bady Ending. Studying alone is not enough to get enough skill points, plus your health will drop and you will get sick. Do study, but also visit Kouichi and Youji a few times in the month, do some gossiping, and rest when your health gets low.