I really like this quote:
Education is an admirable thing, but it is well to remember from time to time that nothing that is worth knowing can be taught.
-Oscar Wilde
but it might be to irreverent for your school.
If not that, then this might work better:
Do not be too moral. You may cheat yourself out of much of life. So aim above morality. Be not simply good; be good for something.
-Henry David Thoreau
or this:
Any life, no matter how long and complex it may be, is made up of a single moment-the moment in which a man finds out, once and for all, who he is.
-Jorge Luis Borges
Do any of those help you?