First, my standard reply to anyone thinking about law school:
NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!
Second, it would be very difficult to flight instruct during the first year of law school. Until you get your feet under you, you will want to give 100% focus to learning the law. During the second year, your free time will depend a lot on what you want to accomplish in law school and after. By they you'll have a handle on the course work, but journals, moot court, finding a summer clerkship, and other extra-curriculars suck up a lot of time. The third year is, by and large, a piece of cake. The coursework is now old-hat, there are less law-realted extra-curriculars, and many people already have a job in hand. I would think that it would be pretty easy to instruct on the side during your third year (if you didn't want to have some law-related part-time job), and maybe even in your second year (if you didn't do the journal/moot court thing -- although some schools require it).
I found this mantra to be very accurate: the first year they scare you to death; the second year they work you to death; and the third year they bore you to death.
Actually practicing law and flight instructing on the side: this would depend entirely on what kind of law you did. Working in a law firm setting, you almost certainly would not be able to accomplish this (at least not until you become a partner or a more senior associate and can push work down to lower-level peons). As someone else said, law firm practice involves too many unplanned weekends and evenings in the office. You would not be able to keep a regular schedule with your students. In other areas (gov't, non-legal jobs, etc.), you might be able to swing it (although my only experience is in a law firm, so I can't say for sure).
I know of a few lawyers in the Cities who also are pilots for NWA. I do not know whether they were lawyers first or pilots first, or how they got to where they are.
Hope this helps.
MF