Im not sure saying you've sinned would even help. Doesn't confessing, repenting and doing your penitence make it immediatly all right?
Unless you're not ready to repent, but that's an even worse lie.
Well, seems like you'll have to suck it up - either your parent's disaproval or your moral objection. If you're not ready to come out yet, maybe you should make yourself some bullshit rationalization like you're not harming anyone and it's all for the better and their enjoyinment of easter or something.
Of course you could use this situation ass a cliff and just fucking jump. I mean, you've been thinking about this for over an year now, how much longer will you take? When you move out you'll have even less pressure to tell anyone anything. If it's hard now with incentive, it'll feel like what's the point later, if it even occurs to you. And maybe you're okay with that, nothing wrong there. But if that's not the case then maybe you shouldn't procastinate so much.
If you're hellbent on neither telling them nor doing it you could always pretend to be kidnapped. There are of course lots of ways out of this, but very few of them are much more ethical then just taking the communion, so what's the point?
I still think letting them know in easy, baby steps is ideal. Of course that's harder with a deadline, but the whole "I gotta seek Truth by myself" act could still work. They'd want to help you and all, you'd give some excuse, say you're gonna go talk to the priest, then you actually tell him your (supposed) situation. Who knows, maybe he can actually help, and it wouldn't be wrong or anything ethicaly speaking.
Truth is you should have taken the opportunity when you had plenty of time to make it right. But I know how procastination whorks, so I'm not judging :P
