your transcript is not a problem... but... you may still have a problem.
juilliard will not care about what you are doing in your school music classes. those are general classes for everyone. you should be involved in extracurricular music specialization activities.
you should be in a youth orchestra. and see if you can find a precollege program for theory, eartraining, history, chamber music, etc...
at juilliard:
they will not care much about your GPA.
they will not care that you played 7 other instruments.
they will only care how well you play the instrument you audition on.
no one gets to be 100% sure of getting in to juilliard.
there are more talented students than there are spaces in the school. so every year, many kids that are good enough dont get in.
there really are no guarantees.
at your age, to be on track for juilliard, you should be playing major concertos and the harder bach cello suites, possibly some of the violin sonatas/partitas.
if you are playing stuff like telemann and 1st suite minuets, you are probably not on that track.
if you are playing stamitz and have played a few complete suites, you *might* be on track.
if you are playing walton and fugues from bach's violin repertoire, you are heading in the right direction.
(if you have no idea what i was talking about in the last 3 sentences, you arent at all on track)
but please realize juilliard is not the only good school out there.
when inexperienced students show up here asking about juilliard... we usually assume its because it is the only famous school they have heard of.
if you were really thinking about this properly, you wouldnt be talking about "juilliard". you would be mentioning specific viola faculty there that you want to study with. that is what advanced students consider when choosing schools.
high level viola students who audition for juilliard often audition for colburn or curtis as well. (great faculty AND they are free).
do you even know who teaches there? you should.
now, if you are playing at a level where you might just barely make it into juillard, but on the bottom rung of that school... probably better not to go. you would get more opportunities being on a higher rung at another conservatory. not everyone who goes to juilliard gets a fancy career afterwards. apparently, some become accountants.
even the "best" school is not the best fit for every student.
you need to learn more about whats out there.