Persona 5 uses a variety of social stats that allow you to interact with different systems in the world as well as increase your relationships with others. One of the easiest stats to consistently raise is knowledge. Knowledge helps with strengthening relationships with specific confidants as well as performing adequately in school.
If you have played for even a few in-game days, it should be apparent why knowledge is so easy to come by. Periodically, teachers will try and stump the new kid with a question. Sometimes this is about history, art or even English. If you answer the questions correctly, you will be rewarded with some knowledge points. Your teacher and classmates will also be impressed.
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Most knowledge gains will come from doing things in class. However, you can also take this power into your own hands. Go to the school library after classes are finished and study in the corner. This will also gain you a few points in the guts social stat, although being in public could potentially distract you from studying, lowering your knowledge gain.
Another option is to study at Cafe Leblanc. Lucky for you, Sojiro has no problem with you using the cafe to study, so sit down at a table in the evening and work on school work. If it is raining outside, you will get gain more. Speaking of Cafe Leblanc, there will occasionally be quiz shows on the TV. Answer those correctly for additional knowledge as well.
Around exam time, your friends may also ask you to spend time with them and study. Do so to increase your relationship and your knowledge stat.
You can also read books to increase social stats. You can check out books from the school library, or you can buy them at the bookstore. On the description for each book, it will let you know which stat will be increased by finishing it. You can read books in the school library or occasionally on the train.