You might find that what you need to get you out of your plateau is some resistance training (weights). You do get a good workout for your chest, shoulders and triceps from those pushups at training, however using weights is great to build muscle, which speeds your metabolism up no end!
Try including 2-3 weight training sessions in each week. They don't have to be huge - just make sure you have a decent exercise for each major muscle group -
Legs (Squats, Lunges, Deadlifts, or Leg Press)
Chest (Bench Press, Chest Press, Flys or Pec Deck)
Back (Lat Pulldown and Cable Row)
Shoulders (Shoulder Press, Lat Raises or Front Raises)
If you don't like doing weights, don't waste time doing things like bicep curls, tricep pushdown as they are used to assist the larger muscles in the weights exercises above.
You probably do a decent amount of abs in your kickboxing classes so I wouldn't worry about that.
Make sure you leave at least a day between weights sessions to allow your muscles to repair and rest.
Hope this wasn't information overload for you! If you have any questions feel free to pm me and I'd be happy to help