Hi Joanna, My name is Julianne and I am a Khan Academy Ambassador who teaches middle school math in Idaho.  What a  difficult problem you have!  I have a couple of questions for you-

1. Which standardized tests are adinistered to your students each year?

2. What sort of parent involvement do you have?

3. Does your school have any computer programming classes or INtro to Coding opportunites for students?

I ask the first question because, quite often, the test prep and practice sections that Khan Academy provides can be a very big selling point to parents and administrators who are hoping to see scores improve on tests like the MAP and  SAT tests.  See:

 https://www.khanacademy.org/resources/k-12-teachers-1#copy-of-about-official-sat-practice-khan-academy

and

https://www.khanacademy.org/mappers

I ask the second questions because input of parents is also often a strong selling point for administrators.

I ask question #3 because Coding and Intro to computer programming are one of the HUGE "Side-benefits" that my students were provided with simply because we were connected with Khan Academy.  The Coding and Computer programming topics are exciting and engaging and they can be accessed from the same Khan Account so our Tech teacher (it is just a single period exploratory class) has joined forces with me and together we are able to provide our MIddle School students with a terrific intro to Computer programming.  (Parents LOVE that - so you are helped with item #2 above as well!)  We have a number of students who are continuing on their own and testing out of two levels of the Intro CS classes at the high school level.  That is also really good PR for the use of Khan in our school!

Fighting with administration is never a good idea, but I do think it is perfectly reasonable for you to "build your case."  I would start with some of the case studies that are available on the Khan website.  Like this: 

https://www.khanacademy.org/resources/classroom-case-studies

I was part of a state-wide research project funded by the Albertson's Foundation (in Idaho) that particularly lauded the importance of supporting teachers and their individual ways of using Khan to meet their specific classroom needs.  In my classroom I use BOTH the school provided curriculum as well as Khan Academy (my class periods do not lend themselves well to blended instruction - so I use Khan topics as a supplement to the coursework but maintain a fairly traditional classroom style.)  Every teacher finds new/unique/effective ways to utilize Khan in his/her classroom and I think THAT is what makes Khan Academy such an attractive solution.

I wonder if you were able to find a way to utilize Khan as a supplement to your current curriculum - if the results could "earn" you more and more freedom to move in the direction you feel is best for your students?  And it can't be a bad thing that KA is completely free- so your school could have the added incentive of losing the cost of their current online curriculum...

The bottom line, I think, is this: do what you can for your students.  If you feel that KA is a more effective means of helping your students learn- find ways to use that are within your ability and time constraints as well as being acceptable to your administration.  But continue to thoughtfully approach your administration with the well documented reasons that Khan can provide a student with resources that reach far beyond their single year of math they may spend with you.