Change: How far do I go?

The problem with change, at least how it is envisaged by universities, is that it must be systemic. As a classroom teacher, I have a certain amount of flexiblity. The Prescribed Learning Outcomes are, for example, deliciously vague:
Life Science: Cells and Systems
It is expected that students will relate the main features and properties of cells to their functions (BC IRP, p. 30)

I can get my students to demonstrate this in any number of ways, posters, prezis, PowerPoints, essays, stories, and so on. However, if students would rather learn about astronomy instead of cells, then too bad. I also have to give students letter grades instead of mastery or comment-based reports, and I must teach to a bell schedule. In fact, students have become so used to this system that they are often confused and even irritated when I try to incorporate Math into Science, or LA in Socials, etc.
Research from the Ivory Tower would suggest that students would learn best if they were allowed to follow their own interests, but that paradigm shift is not feasible for an individual teacher. Why must change be systemic? Is the system, which has taught millions of students in virtually the same way over the past two hundred years, so broken that it is beyond repair? What can a teacher do right now?

For the past six months, I have had students work on a website called Khan Academy. For those who don't know, Khan Academy is a website whereby individual students can practice virtually every Math skill necessary for high school completion. There are videos and hints for when students get stuck, and "points" that can be gained for mastery exercises to keep students motivated. I use the site to allow all students to work at their level. For some of my Grade 8 students that means working on double-digit multiplication, and for others it is graphing quadratic equations. It could be the closest to John Dewey's student centered learning that I have acheived, and it is within the current system. I think that a systemic, paradigm shift is not necessary, what is necessary are teachers who are willing to engage in a fearless experimentation.