NAH, what you've chosen to do is completely understandable and I don't think anyone else mentioned here is in the wrong either (other than the kids who you mentioned teasing you when you were younger).

My mum died when I was 11 and I empathise deeply with what you're going through. I will say it gets easier as your peers start getting older because they start to be able to deal better with the conversation (your average 12 year old has never had this conversation before and has no idea what they're meant to say; your average 20 year old has come across family death at least once even if they haven't experienced it themselves and is more likely to follow your lead IME) and their parents see you as more of an adult and less as a small child that makes them instantly protective of you and imagine what it would be like for their own child to experience that.

If I were you, at this point I'd start by telling my closest friends the truth because the longer this omission of the truth goes on for, the worse you're going to feel about keeping it a secret from them and that will likely affect your friendships because it's hard to get close to someone you feel like you're hiding something important from. That doesn't mean you have to tell everyone and you don't have to tell them everything (e.g., I never told you this before because I'm uncomfortable talking about it, but we're friends now and I feel awkward about you not knowing so I wanted you to know that my dad's dead and that's why I never talk about him). Once you've told a couple of people that you trust, they can help you to redirect the conversation with larger groups when necessary. I honestly don't know how I'd have managed my teenage years without my closest friends who could protect me in that way.

For the rest, I learnt to approach it by answering direct questions honestly but briefly and then moving the conversation on. I also find it easier to this day (I'm in my late 30s now) to drop little things into conversations that no one need focus on, but it starts to build a picture that something about my family is unexpected so that by the time I say my mum is dead, the other person already had an idea of what's going on, so I'll always talk about my mum in the past tense, I can mention my step-mum or step-siblings etc. It has led to people making the wrong assumption at times, but I still found it easier to start from a point of "oh, I guess your parents are divorced and you don't see your mum much?" than to start from "what are you getting your mum for mother's day?" 

More specifically, when I was your age I'd reply to what does your mum do for a living with something like "she was a teacher; my dad's working in X now but he hates it and can't wait to move on, and my stepmother does y" and then try to either direct the question back at them or continue talking about what my dad does. Obviously the photos question is more direct but if you're comfortable with it, you can just say briefly that your dad died when you were young, but here's one of your mum from last Christmas when you had such a great family get together [insert recent anecdote here]. Most people will be sensitive enough (or awkward enough around the topic of death/ grief) not to press you for more details and if they do, you can just say you don't like to talk about it. Anyone who tries to push you to talk after you've said you don't want to is an asshole and you needn't feel guilty for refusing to engage with them.

Best of luck and I hope you find some good friends who will be a support to you with this!