Yes, I believe you are considered a resident. I have done a lot of looking into residency requirements for multiple states. And most states go by either where you file taxes or where your parents are if they claim you as a dependent on their taxes (which would depend on where they file their taxes).
The only exclusion is where you are in the state for the purpose of school (college) but since you went to highschool there as well you are fine. Being registered to vote is one of those additional tidbits they look at favorably.
If you file taxes in IL then you likely claim that are your permanent residence on all paper work for schools/everything since you came here when you were a teenager.
Not having a drivers license will not hurt your residency. I imagine you do have a government issued ID, and hopefully that is under IL.
And remember if by some freakishly slim chance they do declare you out-of-state, you can always appeal!