I'm going to get a drink after a final and not tell my so
Why are you getting a drink after your final?
Because I want to spend some time with my friends
It is normal to want to celebrate after you completed something big. Lying to your so wasn't great, you should just tell her the truth.
Young fun loving guy living for today
It is normal to go out and have a drink with friends to celebrate. You should talk to your SO and explain what you are doing so they won't feel hurt.