It appears that the ABA has quietly dropped the limitation that full-time law students only work up to 20 hours per week.
Before the latest iteration of the ABA Standards for Approval of Law Schools, this was the pertinent standard:
Standard 304. COURSE OF STUDY AND ACADEMIC CALENDAR
(f) A student may not be employed more than 20 hours per week in any week in which the student is enrolled in more than twelve class hours.
For 2014-2015, Standard 304 is now Standard 311, and there is no mention of the 20-hour work limit.
If you are interested in working, beware that while the ABA seems to have dropped the limitation, law schools may still impose the restriction through the honor code. If your law school does have a 20-hour limitation, you may want to discuss it with the powers that be and notify them of the change in the ABA Standards.
And although you may be able to work more than 20 hours per week while going to law school full-time, it may not actually be advisable. Law school is supposed to be an intensive course of study, and a full-time job will inevitably take away from the immersive experience that is recommended.