i'm a high school student who recently joined an ensemble who is playing sonatina I by mozart,
and i received first part.
the only problem is the upper range.

i can play the C's and D's above the staff but my problem is i have to do chromatics up to them,
because every time i try to just play them, my instrument kind of growls but doesn't sound the note,
or it won't sound right away.

there is still a few months until we have to perform, but i need help with what i can do to make those notes sound....
not so horrible.

any suggestions would be fantastic

When your instrument is in good playing condition the problem you describe tends to be due to the reed. Change your reed, not necessarily harder or softer, just a new one. If you continue to have this problem, especially if a technician says your horn is good, try using a Legere plastic reed which doesn't change due to drying, humidity, vibration, etc.
Good luck .

If the bass is working well & you have a good reed possibly the problem is not using enough breath support & squeezing the reed too much. It will take practice to get the hang of the difference between the soprano instrument & the bass.

Might sound a little obscure, but if you can't get the sound right away try smacking your fingers down on the finger pads...I know it sounds weird, but its always worked for me...especially upper register and lower register notes...

Good luck

Most student bass clarinets leak quite a bit. I'd get it looked over by a professional tech.