If I'm not mistaken, the ABA has a requirement for approved law schools about how many hours a student must be in class. I know at my school you're allowd to miss 1 hour of lecture for every credit the class is worth... so generally, you can miss 2 class sessions. Exceeding this can be groudns for being denied the opportunity to take the final exam, although I've never heard about that actually being applied or enforced. Some teachers took attendance each day by sign-in sheet, others did it by observation and seating chart, and yet others couldn't care less.
The real impact is on what you miss by not being there. Sometimes things will be discussed in class that aren't in the reading assignments - 1 of my 4 Torts I essays was literally a restatement of a case the teacher talked about on the 2nd-to-last day of class. It wasn't in any of the assigned readings, nor was the case name actually given to us... but he laid it out plain and clear and explained the reasoning behind it. For those of us in class paying attention that day, it was an easy question - but if you weren't there that day and didn't get good notes from someone else, it would have been a nightmare.
Missing class here and there when it's unavoidable happens... but as long as you're only talking about missing a minimal amount and as long as you're diligent in getting the notes and reviewing the material, it's not the end of the world.