。
She doesn't have to go to school on Saturday.
Literal
She [topic-は] Saturday school [to-に] go-not [even if-ても] good.
~なくてもよい is the slightly formal version of ~なくてもいい ('don't have to / it's fine not to'). よい is the base form of いい — using it here gives a slightly more careful, written feel. The pattern negates obligation: she has no requirement to attend. Note the omission of the particle に after 土曜日 — it's optional with time expressions.