xuesheng refers to the identity of students and it's usually used to show the status of a person. when teachers greet with students, we usually call students tongxue. with regard to tongxuemen wanshanghao and wanshanghao tongxuemen, they are completely same

3 or 4 is better, we use tongxuemen when addressing students, xueshengmen sounds like you're talking about him/her/them.

3 同学们， 晚上好!