It really depends on your school policy. At my school it is an honor code violation and you have to sign a statement each semester when you register that you will not work more than 20 hours if you are going to school full time. If they suspect you are working more than 20 hours for any reason you must prove you are not. If you can't prove that then you are automatically placed in part-time status that semester and you will be withdrawn from the number of courses it takes to make you in compliance with part-time load requirements. It is the school's discretion as to what classes they drop you from. If they discover that you worked more than 20 hours a semester after grades, they will still withdraw you from the number of courses it takes to come within part-time requirements and you lose those hours.
In addition, because it is an honor code violation, you are subject to sanctions for violating it too. And the 20 hours work requirement is throughout law school, it's not a 1L thing.