Well, it has been awhile since I looked at art school application requirements, but I remember that most schools gave portfolio requirements and some were very specific about what type of artwork they wanted. So, I would check with the schools you are interested in to see what they want to see in a portfolio; check their websites. If art school does not list specific types of artwork, then I would choose a variety of your work to show them--traditional drawings and paintings, computer artwork, photos, prints; whatever you have done, all with a variety of subject matter. I also suggest you talk to your current or previous art teachers to see what kind of work their students have submitted to art schools in the past, and where their previous students have gone to art school. I also strongly suggest you show your portfolio to your art teachers for feedback.
Whatever you submit, select your best work that shows your skills AND creativity. Don't get too caught up on perfect technique, that is something that will come with time and experience. Try to show that, on top of your art skills, you are a creative thinker who is not afraid to take some risks. Also, I would try to communicate the idea that you are potential art student who is ready to work hard, and who is open and eager to learn. Remember the great French artist Henri Matisse's famous quote: "Creativity takes Courage".
So follow carefully the requirements of the schools you are applying to, and show them what you've got and what you can contribute to their school. Best wishes!
It's good to see that you're thinking about what to put into your portfolio for art school applications.
The answer is actually pretty complicated and depends on the school and major for which you are applying. Fortunately, most schools will have specific guidelines for the various majors listed somewhere on their website.
For example, if you're applying for animation you would include a demo reel that shows off what you've learned and done so far. Alternatively, if you're looking into illustration you'd include a variety of pieces that demonstrate a variety of subjects: landscapes, still life, figure drawings, etc.
Schools don't expect you to already be an expert artist. They're generally looking for potential and some basic aptitude.
I hope this helps. Again, make sure to check with the schools' websites to find the specifics. You might need to prepare a separate portfolio for each school.