1. When I was at school this was a generic term meaning "to pair off with", usually used at a party or other event designed specifically for pre-pubescent pre-sexual encounters. Means anything from a kiss to "going all the way"
2. However my other half thinks it means specifically "to have sex with" at the same event that you meet the person.
1. Oh my God! You'll never believe who Sally got off with at Dave's party last night!!
2. I see you used all the condoms up, you must have got off with Dave last night then?