Normally when this situation happens, things start crumbling down. Leading to a breakdown in the relationship.
I say this because I have seen it with my class mates.
And besides the question is are you considering the possibility marriage? Do you have a plan and goals to complete it? Usually a relationship should not last longer than 2 years because of the potential risks at the same tine you do not want to rush it too.
Look go to your spiritual father, a catholic priest, explain him everything but only with the truth, because if you do not then you commit sacrilege which is another level of sin worse than mortal sin, you do not want to go on that path. I am not saying you are, I am just saying that you want to tell him the whole truth.
Once you do that follow his advice and heed to it.
You should be the man and talk to her parents let her know you want to ask her parents.
Whatever happens she must say the truth. You as the man and leader must tell her that she has to tell her parents only the truth. If you truely love her you want the best for her and if you are condering future posibility of marriage it best if you tell them.
In case your parents do not know they also need to know even though you are 17.
Now if you cannot marry her within the next 2 years then why go out and play with fire?
Wait till you have the ability to marry her.
In high school even though most of my friends had girlfriends I did not see the point of me having one although I did have a desire so instead what I did was pray for her for all this time. I am now 27 and now have my first official girlfriend who I am considering marriage.
And of course hiding your relationship is bad.
You have to tell them. It is going to break their hearts that you hid it.
But precisely because you love your girlfriend and want her to be truely part of you and your family you must tell your parents, otherwise matters will get worse and then they blame the girl etc etc and it gets to a very deep mess.
I mean you are already into trouble but it still can get fixed by telling them the truth.
If you do not know how to tell them nor how to approach it tell it to your priest. Tell him you want to have spiritual guiadance.
You can also ask Our Lord throught he priest in confession whether it is bad.
He will tell you but be extremely transparent and honest, remember you are speaking to our Lord in spiritual direction or confession on that moment. That is who the priest represents.
Right now things for me and my girlfriend is going smooth and it is all due to Our Lady in the Holy Rosary and God.
Our Lady will guide you and help you make the right things.
Take her to places in places where it is healthy spiritually for you and her.
Please follow whatever your priest says.
And remember honor your familys name if it were not for your parents love you would have not have existed.
If you tuely love her make sure she is ready for heaven at any moment by your actions. You are the elader do not spoil it more than what it already is.