I teach high school art. Hi Paint, One way to get great ideas for H.S. portfolios is to contact art schools around the country and request catalogs.
Many schools have specific admission portfolio requirements that are stated right in the catalog.
Also, most schools have very lavish spreads of color photographs of student work examples. A few schools will even send you a video about their school at no cost to you. This will give your students an idea of the expectations of art schools and give them an instant feel for what art school is all about. In the meantime, your students should be collecting a variety of their best work. If the portfolios are to be seen in person during an interview, the work should be matted and professional looking.