Formal:
* Elle a étudié ce week-end. (This is the most literal translation and is appropriate for most situations.)
Informal:
* Elle a bossé ce week-end. (This is more casual and means "She worked/studied hard" this weekend.)
* Elle a révisé ce week-end. (This is more specific and means "She reviewed" her studies this weekend.)
More specific:
* Elle a étudié ses cours ce week-end. (She studied her lessons this weekend.)
* Elle a travaillé sur son projet ce week-end. (She worked on her project this weekend.)
The best choice depends on the context of your conversation.