Talk to Jacob. Ask about his flower. And tell that you know why he has it. ? Ask him to give it to you.
You have to read the biology book, first.
He has passed many messages to Jannike. Has she read them? Ask Jannike about the letters.
Then they must still be in her desk.
Jannike's desk is the closest to the heater. Open it and click on the stuff. You will find the letter.
Give it to Jacob.