The word ain't is no longer in common use by people with basic education. It might be in use regionally, but the people using it know it is not 'proper' English. It's an affect or slang and used with humour or in relaxed circumstances. I might use it in the same company where I would feel comfortable using a swear word. If you were joking, please excuse me. I completely understood what you meant.
"Should I resign from the Student Organisation next?" is fine, but I think I would say, "Next, should I resign from the Student Organisation?" It make the question more clear or concise -- we know exactly what you mean to say.