Would I get instate tuition if...?

Did you move with them or are you still in Missouri? If you moved with them, you are fine. If not, it appears that if they claim you on their taxes then you can get in-state tuition. She really can't be "one of your legal guardians" ... it sounds like she is the noncustodial parent.

Dependent students:

The residency for tuition purposes of a student who is not independent is based upon that of the parent or court-appointed legal guardian who claims that student as a dependent or provides more than half that student’s financial support, regardless of the length of time the student has resided in Texas. If the parent or court-appointed legal guardian of a dependent student meets the criteria of having established residency for tuition purposes, the dependent student is eligible to pay resident tuition. Parents and legal guardians qualify for residency following the same criteria as independent students.

I'm pretty sure you need some type of address that shows that you've lived their for 12 months. It might suck, but you might have to move in with your mom your senior year. A good thing to do is to call Texas A&M admissions and see what they say...

Prospective Student Center

109 John J. Koldus Building

1265 TAMU

College Station, TX 77843-1265

(979) 458-0427

You can try using your moms address to show that you are an in-state resident however when you send in your high school transcripts to the college they are going to see that you have in fact been living full-time in Missouri.

If she claims you as her dependent when she files her taxes than you should be able to get in-state residency. But contact the school to verify this.