@Pzaz99 — I agree with just about everything others have already said — I’d just want to add one more thing.
This depends somewhat on the circumstance of the guardianship – but if this was not a planned thing you also may want to consider whether you are sending an unintended message to your niece by focusing too much on plans for college. You may be thinking about her future, but she might be interpreting your enthusiasm for college prep and planning as being part of a desire to get rid of her as soon as possible.
That is, unlike your natural children, she has no assurance that you will continue to provide for her or care for her after she turns 18. For most teens, the thought of leaving the parental home and going off to someplace new is an enticing adventure – and one that comes with an unspoken safety net, of knowing that they can come back if things go wrong.
But your niece may not have that feeling of readiness for a new adventure, or sense of security.
I also do realize that you may not be able to make a long-term, open-ended commitment to provide for her. Perhaps you have your own economic limitations and really do need her to be ready to launch after she graduates from high school. If so, that is one more thing that needs to be understood and communicated clearly. But again, you are dealing with a teenager who may need a lot of support and reassurance along the way. So if your home will be open to her post high school – let her know that now, so she doesn’t feel like you are impatiently counting the days until she leaves – and if not, then find a way to help her ease into consideration of various options or alternatives that she may face at that time.