Demonstrate your willingness to experiment
Depicting a wide range of subject matter throughout your entire art school portfolio demonstrates your willingness and interest to work in many contrasting ways. There is an endless range of possibilities when it comes to subject matter in a portfolio.
Admissions officers don’t want to see an art school portfolio of twenty self-portraits. When a portfolio has too much of the same subject matter, the message you are sending an admissions committee is that you don’t want to try anything new, and that even at this early age, you’ve are already set in your ways.
Attending art school is all about exposing yourself to as many new experiences as possible. Having an art school portfolio with only one topic comes across as narrow minded and limited.