Placing the irises in the exact center of the eye, with the rim on the iris touching the edge of the eyelids makes for a rather intense, focused and maybe scared expression. Eyes are usually not that wide open, and tend to cover a large part of the iris, often to a much bigger extent than I've drawn here as an example.
I know you have started experimenting a bit on your own, on some of the drawings you've done lately the eyes have been less wide open than before. On your latest drawing I see you've also added a reflection in your eyes (and on the hair), which is a very good way to make them look more three-dimensional.