Oh you can find the clues in various places, like for the first password to open the diary, its from talking to an npc near a bench in the shops, for the first diary entry, it's from meyza's nickname to you during your conversation, the second diary entry is mentioned by the MC multiple times in Io's arc and in the potion classroom, third is when MC meets the principal the first time, in the guide, I just state the passwords as is since basically the answers are all already stated as you play