"The pencil is short."
it is possible, and it is a more casual verb ending. the "습니다" that duolingo uses is the very polite form used around people much older than you, around strangers, and in formal situations, and the "아" is the casual form used around people younger than you. the "아요" you will also eventually encounter is used around people you know, but are older than you
shouldnt they use the topic marker "은" rather than the object marker "이"?