As an art collector here are my views: (please excuse my honesty)

Piece one: usually when a waterfall shows that color there is a push behind the fall therefore the stream itself should look quite faster. the rock to the right of the waterfall look a little more melting than an actual rock. I love the way you have the full bottom layer of the rock going the same way as the stream but that goes back to my point of where is the waterfall coming from especially pushing down that fast (thus white water).

Piece two: you should have been listening about the color wheel in class.

Piece three: the butterfly reflection is a 2 out of 10 (honestly), where are the ruffles of the reflection butterfly from the stream of water, the shadow of the leaf shouldn't be where it is from the angle of that view.

Piece four: You should "experiment" more with water colors I think this is where your talent can be shown the most. Because you have to work on your real life details as stated in Pieces before. I love the definition and emotion of her hands that really bring about what this piece is saying. Her inner shoulder area can use some work , where she seems like she's standing straight instead of turning. The waist area is missing correlation with the knee area.

Piece five: Is this turtle caught in a net? A near miss? the background let me know this turtle is swimming quite fast. I really cant get a good read on this piece which is not a great sign (you know this hopefully). the shell of the turtle upper left shoulder area is too open to not expose skin, instead you have darkened the area. I would have to give this piece a 4/10.

Piece six: The habitat isn't "lost" until the fire is completely gone. The chimp shouldn't look as it have a "scared" emotion it should mimic more like a slight panic. The backgound is brilliant.

Piece seven: If you look at the head area on the left (her left) side next to the temple, it is a little too inflated. WHAT IS IT WITH YOU AND THE LEFT SIDE OF ART PEICES. The "hoop" nose ring shouldn't pull her nose unless the just recieved the piercing. (highly doubt). What emotion is she showing , where is the expression.

Piece eight: The iguana is a brilliant piece, I see the detail, all of your art in the future should have this piece as a minimum standard. Future advice, if your eyes are poping out STOP AND COME BACK LATER, it will help. A branch will help a little on this piece, but i see what you were trying to accomplish here. 7/10 for this piece.

Overall: 6/10