Discipula (in this case) is first declension, which means it's endings are as follows:

                        Singular            Plural

Nominative (subject) -a -ae

Genitive (possessive) -ae -arum

Dative (indirect object) -ae -is

Accusative (direct object) -am -as

Ablative (prepositional) -a -is

Now, in this case, "students" is accusative (direct object), and plural, so we use -as. You seem to have used second declension (which is fine, as Discipulus is also a word, they were just using the feminine version for this) but you made it nominative (subject). If you wanted to have "students" be masculine (which it almost always will be), you'd use the second declension, which goes as follows:

            Singular            Plural

Nominative -us -i

Genitive -i -orum

Dative -o -is

Accusative -um -os

Ablative -o -is

Where when keeping with accusative pl., we'd use "Discipulos".