Yes, you must officially request that the school evaluate your child in writing under IDEA. Even though you have a private eval, this is how the process starts. You can submit your private test results. You write to the principal. This triggers the IEP process, but the 504 comes along for the ride. There's a web site called Wrights law that has a good intro to all this.
The big advantage of the 504 for us has been to make it official with the teacher - not just an odd child, one who has a reason for accommodation. Also, testing accommodations for all the various computer tests as well as in-class assessments - extended time, quiet space, etc.