She can use your address. But you do not want to claim this time as time spent living together since she is only there on weekends. If you want to sponsor her for PR, you need to be either common law or married. To be common law, you need to live together continuously for at least one full year. If you go the common law route, you don't want to start the one year clock until she has finished school and actually moved in with you full time. If you try to count this period towards common law and say you're living together, you could put her at risk for misrepresentation. So long story short, no issues with what you are planning as long as you understand this period cannot be counted towards common law.
