"Mum, Dad, I am quitting tennis."
If further explanation is required, you could say something like "I've been playing for almost 8 years now and have spent so much time and money into the sport. I just don't seem to enjoy the sport anymore( have been feeling this for almost a year now- just didn't want to go for coaching sessions, etc). I'm in year 11 now so it's hectic with my tennis and other extra curriculars going on".