How did Annie Sullivan graduate from a school for the blind?

I'm doing a research project on Helen Keller, and I put info. in that Annie Sullivan graduated from the Perkins Institute for the Blind. I have to do an oral report, and I would like to be able to explain that, and I'm also wondering myself! It's just that it's confusing that she has eyesight, and would be the valedictorian of a college for the blind!!