Koro-Sensei Teaches Many Lessons in Assassination Classroom

"Whether a fish lives in a clear stream or a water ditch, so long as it continues swimming forward, it will grow up beautifully."

Koro-sensei brims with inspirational advice, but this quote in particular resonates more than others. It shows no matter how difficult your initial circumstances may appear, if you keep going, you'll eventually arrive somewhere better. This is something his students - preteens with a variety of personal challenges - truly need to hear. This applicable outside the anime, too.