You can get SSN once you get on-campus job, it doesnot matter on which semester you are, sometimes the SSN process might also depends on which college you are applying for. If you dont get a on-campus, as a law a student can start working after 9 mths of continue studies excluding Summer break, if you take class in summer you are eligible after 9 mths. To get SSN after 9 mths, you need to get a job offer that compiles your course of studies.