nouns that start with z~ follows the article "lo" is true. On the other hand, in order for nouns with "s" to have follow the article "lo", the next letter after the "s" MUST be a consonant; "lo" sTudente. In the case of "sErpente," there is a vowel after the "s," for that reason it is "il" serpente instead of 'lo" serpente.
Z = lo : zucchero, zoo
S+ consonant= lo : stadio, studente,
Consonant = il : il serpente
vowel = l' : l'orso
hope it is helpful to you.