Top of Class requires you to get the kills in the daily focus playlist. On Tuesday the focus playlist was strikes. At 1700 UTC yesterday the focus playlist switched to Crucible. And today at 1700 UTC it will switch to Gambit. If you have been playing strikes for the last 20 hours then that is why kills have not been counting. You need to be getting the kills in Crucible up until 1700 UTC today. Then you'll need to switch to Gambit. The focus playlist can be identified on the Director by the Guardian Games symbol appearing next to the playlist icon.