Those who never believed in a God

[Der/die AtheistIn]

The thing is I'm about to begin the last year of High School and take the baccaulaureate. I also plan to study in Vienna and I need to take an exam for it (it's not hard, but I still need to practice). My parents bought me an appartment in Vienna, so that I wouldn't have to stay in a student housing. My dad may be a little to religious and short-tempered, but he is willing to help me get a good education.
The idea is, we have a lot to do and I don't think it's the right time to talk to my mom about my atheism. And even if i would talk to her and get her permission, my dad would still forbid it. My mom would either try to convince him otherwise and fail or not try at all. 
I would prefer to talk to my mom after I get to live on my own feet. I don't know about my dad, though.

Your intuition is likely correct that this is not the time to challenge your parents with full knowledge of your apostasy. You are young and have plenty of time to establish yourself. Play along, and make the break later on, when maybe they won't connect the dots or wonder if they could have / should have been stricter with you. Needless confrontation doesn't prove anything.

At least you are not living in the US Bible Belt where you might well not be allowed to attend a quality secular university at all, or where your parents would be affirmed in being even more controlling.

Focus on what you have (loving parents who are helping you get educated) rather than on what you don't have. The former is way more important than the latter.

At your age it's easy to feel impatient about being differentiated / independent of your parents but the truth is, there's no huge hurry, not when they are buying you an apartment and being so supportive. The rest is unimportant details, actually. You'll have plenty of opportunity to demonstrate that you're your own person later on.

Thanks.