Short novels are called novellas. If you don't want to write a novel, but have more to say than a short story, you could try writing a novella.
You've surely heard of plays, poems, short stories, and novels. But novellas? They're not as well known, but pretty simple: nothing more than short novels. Sometimes novellas are called novelettes, but whatever you call them, you'll probably read a few in English class. Sometimes it's easier to squeeze in a novella than a whole novel, so maybe you could start with Joseph Conrad's "Heart of Darkness" or Franz Kafka's "Metamorphosis."