Very nice
I love what you did with the background in serenity
a few minor things though.
you are very liberal with your use of white, be carefull of that, especially when doing trees, white is not a colour that comes natural in trees, so using it as a highlight on leaves is not advised, makes it look very pastely and fake.
your sense of light and dark is great, especially in the first one, but your sense of where its coming from is not, your light is backwards in the first one, the ground and background say its coming from one direction, but the lighting on the leaves says its coming from the opposite.
also a few issues with basic shaping, understandable given how long youve been painting, your trees look stretched/oddly shaped.
The final results here are quite great though, as good a landscape as one would expect to see in any highschool final art exam.
although i do think you were saved by the fact that landscapes are a very forgiving subject and oils a fantastic medium.
If you're going to stick to landscapes, i would suggest doing something a little more defined, like the tree one, but with more in it. let the reason it was great be because of you, rather than the subject and medium.
would also suggest writing your thought processes down, to help solidify your concept and doing individual practice pieces, on paper or something, to test out colours and on some of the more difficult areas