It all depends on your location.  In most major cities there will be a selection of schools to choose from.  In some you can access them easily by public transport (e/g/ New York City); in other cities, you'll need a car (e.g. Los Angeles).

One tip.....open up the yellow pages -- you'll see lots of dance schools, some of which offer ballet.  Look for a school where the "director" has had classical training....often indicated by phrases like "RAD," "Cecchetti," etc.  AVOID schools with names like "Little Miss Missy's School of Dance and Baseketweaving," 'cause they're unlikely to have a big adult class.  Try places like "Academy of Ballet," and those that indicate they have a performing company.

One other important thing to look for is DANCEMAGAZINE, a US publication that has local ballet schools, listed by state, in the back.  It's expensive to advertize in this listing, so the schools are typically "serious" and well established.

Good luck!  BalletMaster