Before you lie to avoid it, think about it. Are you sure you want to pass up an event that can actually be fun? It really depends on the reason/s why you don't wanna go. But if you really are not comfortable, you have the right not to go.
I dont want to sing and I want to avoid the school concert which is in one week. What should i do? Should I lie?
Do not. Go anyways. What do you have to do that's so important instead? You gotta grit your teeth and do what you are obligated to. It's an important skill to do work at things that you don't want to do and to endure things that are unpleasant.
Is it that you dislike singing? Hmm. I've never met anyone who disliked singing...