I have trouble controlling my dynamics on the flute, especially on high and low notes. People told me that if you blow harder, the note will get louder, but when I try to do that on low notes, they always jump up an octave,(vice versa on higher notes). I'm trying to learn this so I can sound expressive when I play. Thank you!
Argh, those infuriating high and low notes! I know how you feel, and while it is true that the low notes will never be that loud and the high ones will never be that soft, there are ways to make them more even.
Blowing harder is not the answer to make the low notes loud - as you noticed, that just makes the note jump up an octave! What you need to do instead is focus your air, by tightening your embouchere (your lips) and directing the air downwards into the flute. As a matter of fact, that is the correct way to make your notes loud, no matter their range; blowing harder is a waste of air and tends to give you a squeaky sound.
In the same way, relaxing your embouchere somewhat and not directing the air in the same way will help to make the high notes softer. It's a shame you don't have a flute teacher to help you with this, but if your band director plays trumpet, he probably knows something about emboucheres and dynamics (though the way trumpeters use their embouchere is quite different from how flutists use theirs). You could talk to him.
Keep trying! When you get it right, try and notice what you did; then you can repeat it. It will come :)
Practice. Particularly practice long notes, crescendo, diminuendo and at steady levels. High notes can be played pretty softly. Low ones can't get THAT loud. Your teacher will demonstrate and point out any basic technique problems.