It seems to me like the OP has a free option here (or at least a very cheap one). If the OP’s goal is to gain admission to UCLA, then what is wrong in applying in the junior year and retaining an option to apply again in the senior year?
The OP has a rare gem of that opportunity. I.e., if one gets accepted (and will know that by mid march) then one goes. If one gets rejected, then one simply stays in high school and applies the following year.
The OP gains experience in creating a good application. There are little downsides to doing this.
Why is everyone advising against it?