You are enough

A+
Excellent interpretation

I disagree.

Saying someone is enough in the way presented here indicates they are perfectly fine in the state they are currently in.

I believe the author of this was trying to impress on the students that regardless of how they may score on the upcoming tests, they are already a person of worth.

If so a student with thinking power would inquire: If so, why I have to attend school? Why do I have to spend money and time here to learn? Seen in this way, the teacher would sound to be a liar.

You’re making too much of this.

He was trying to calm kids down over the pressure of taking a test.

Don’t overthink it.

I'm pretty sure this is the exact same letter he pulls out once a year at the end of term, sending it to his students so he can feel good about encouraging them.
This letter is a momentary blip on the students phones.

Anyway, if some student does ask themselves why do they have to continue education and spend tons of money, that might be a good thing.