Ze is the unstressed form. Zij is the stressed / emphasised form. Both are correct.
Zij draagt een rok. is emphasising that it is she who is wearing a skirt (and not him or me).
Ze draagt een rok. is a more neutral form. Depending on the context and intonation/stress placed on the word rok (when spoken), this sentence could mean that she is wearing a skirt (and not a dress or a diaper).
Ze - Feminine Pronoun, Unstresssed ( She ) •
Zij - Third person singular Feminine subject pronoun; ( She )
Third person Plural subject pronoun; ( They )
Hij - Third-person singular, Masculine, subjective: ( He )
Het • Neuter Pronoun ( It )