This is the situation i'm in, my parents know i go to a boxing gym for fitness only, but i want to start boxing as an amateur, i'am 100% sure if i try to tell them they will not let me do it, so, is doing it secretly Will be bad? this is really annoying as what happens if i get a cut or something then have to explain what happened... :/
if you are over 18, make your own choice. if you're under 18, then you are putting your coach/gym in jeopardy if you do get hurt (even though it's the gym's responsibility to make sure you have parental consent, lots of gym owners are good people trying to help kids out, but not necessarily good at business). when a responsible coach is taking care of you, you'll probably avoid any serious injuries. Minor ones will definitely happen though. If you don't tell them, they will find out. if its a good gym, consider inviting your parents down to watch a training session and talk to your coach.
Who are we to tell you what to do? Why would the opinions of complete strangers matter in a decision that only you can make?
Back in the days when I was fighting my mom found out about me fighting by reading the papers. She didn't like it, but since I won .... no problem.
How old are you, and do you still live with your parents? Knowing those things will help people advise. Oh and what country are you in?