Learn German: Bringen Sie die Hausaufgaben morgen mit! - Bring the homework with you tomorrow!

This is the formal imperative (a command using the polite Sie form). To give a formal command in German, you simply take the standard present tense sentence (Sie bringen) and swap the verb and the pronoun so the verb comes first: Bringen Sie... !

Why is the word mit all the way at the end of the sentence?

The verb being used here is actually mitbringen (to bring along / to bring with). It is a separable verb. When you give a command, the main part of the verb (bringen) goes at the beginning, and the prefix (mit) gets kicked to the very end of the sentence.

The English translation says "the homework", but isn't die Hausaufgaben plural?

Yes, exactly! In English, "homework" is an uncountable singular word, but in German, it's very common to use the plural form die Hausaufgaben (literally "the homeworks") to talk about your school assignments.

How would I change this command if I were talking to a group of my friends or classmates?

If you are talking informally to a group of people, you use the ihr (plural "you") form. To make this an informal group command, you just use the standard ihr verb form (bringt) and drop the pronoun entirely: Bringt die Hausaufgaben morgen mit!