No guarantees, of course, but FWIW, my dd13 scored very highly on the WJ-III at age 7 despite no advanced instruction. The writing portion was at grade 18+ and the reading part a bit lower, but pretty close to that. She had had virtually no instruction at all in reading or writing up to that point b/c all they'd taught in school was phonics, reading early reader books as a group, and maybe some very minimal writing instruction, but nothing beyond basic punctuation and spelling.
I've seen the WJ-III achievement tests that my ds took at age 8, and they really didn't include much more than very basic concepts such as putting together very short sentences or linking a picture to a concept, things like that, things that you would expect a child in the grade level he/she is in to have been exposed to. You could probably find good descriptions of them online if you search for it.
Another thought - I'd consider talking to someone in the district gifted program to see what's offered outside of your magnet school - you can ask if there are services or acceleration offered in first grade at other schools. It's not an over-the-prinicipals' head type of talk, it's just looking for information so you can make choices for your dd (whether or not you'd ever consider any of the other programs as choices). As part of that conversation, you can ask what testing the district uses to screen for the gifted program.
polarbear
ps - I think once you mention your dd's FSIQ to a district gifted services representative you'll find they are very happy to talk to you! I've always found it's much easier to talk to the gifted program staff than to the regular ed teachers and administrators at our kids' schools about gifted issues - they tend to discount what a high IQ is or how infrequently it occurs, whereas the gifted staff is very familiar with seeing all the borderline kids who's parents try to advocate their way into the program. (Oh gosh, I apologize for sounding a bit snarky there - I really don't mean to!)... Anyway, the gifted services folks will know your dd is really bright and understand that she's bored. It doesn't guarantee they'll do anything about any of it or be able to help, but it's at least always nice to talk to someone who understands
Plus you might also get some info that will help you advocate in future years.
