Is this the same school she's in this year or a new school?
Since it's a small private school, I'd probably take two approaches:
1) Reassurance that she's capable. If she's not changing schools, have her math teacher (or previous teachers) come to the meeting to throw in their opinion that she's capable. If it's a new school, see if they will write a letter of recommendation stating she will be able to handle the work independently. If you can think of other situations in which she's worked independently (like at a summer camp or a course she's after-schooled in or something like that, you can mention that too).
2) Spin this as an *advantage* for the school - something that future potential families looking at the school might see as a plus - the willingness to be able to adapt for special circumstances. We have a similar-sized small private school in our town, and when we looked at them for ds, they were very eager to show off how they worked to individualize curriculum based on student needs. Like your school, they were only able to offer one Algebra II class, for example and one Spanish 1 class per semester etc - but they had students working at many different levels of math - the kids who were the outliers in some cases left campus to take the courses they needed at a local college, in other cases they switched the order they did their science courses in - so that kids who were accelerated in math in 9th grade would take the 10th grade science course, and kids who were not accelerated would take the 9th grade course. I just have a gut feeling that since it's a small school, there are a lot of upsides for the school to accommodate your dd's taking Algebra II independently so she can stay in band and also take Spanish 1, and more importantly, so she can fulfill her academic needs and desires. Those upsides can be turned into selling points for the school - so if you can spin it that way it might help sell it
Best wishes,
polarbear
