Harder reeds should help...some. It's also a matter of breath support. Make sure that when you play, you are blowing warm air throught the horn. Also, use you diaphram to push the air through the horn. To practice this, form your mouth as if you are hissing and try to blow for long lengths of time. Tense up you "abs" and blow from your diaphram, not you throat. > Deep breaths!
I had this problem a few years ago as a freshman in high school. As my breath support developed, that buzz went away. Do some research on it if your band director doesn't have any more advice on the topic.