You must try your oar in deeper water.

You've outgrown me--and I'm a dull old fellow at best.

You must go where you will meet other girls, and deal with a variety of teachers,--not just one dingy old fellow like me.

Have you ever thought what kind of a school you'd like to go to ?" "I don't believe I have; I don't know much about schools." "Well, don't you think you'd like to get away from so much mathematics and learn things that will fit you to be entertaining and amusing?
You know I've taught you a lot of things just to amuse myself and they can never be of the slightest use to you.

I suppose you are the only girl of your age in America who can read the sextant and calculate latitude and longitude.