In the USA, the American Sailing Association (ASA) and United States Sailing (US Sailing, sail sport governing body in USA) certify sailing instructors. (The Red Cross got out of the business of training small-boat-sailing instructors some years ago. The Royal Yachting Association oversees training in the UK including the very thorough Yachtmaster curriculum.)
IMHO, the US Sailing process might be a bit more thorough, but the important thing is differences between schools and instructors, who may focus on different things, differ in the amount of helm
time, have different equipment
(boats), and different instructor personalities. And, of course, different sailing venues vary in their level of challenge and quality of sailing, and sailing opportunities vary seasonally.
Different instructors will have different specialties and certifications. Some may be certified for only small keelboat training, others for only small dinghy
training, some for multihulls, and some for the whole gamut of boat types and experience levels.
Ideally, you would space out your training a bit and get plenty of practice time in between lessons, and get lessons from at least a couple of different instructors. Variety is the spice of life and it's far better to sail on different boats, in varying weather
, with different skippers, that to do the same kind of sailing over and over again.
The best match between you and a school will depend hugely on your sailing interests/goals, abilities, and limitations.
Without that knowledge, instructors can't do a good job of teaching you -- because sailing is something you ultimately have to learn for yourself, even with the help of others.
If your goal is chartering, you will probably learn that charter
companies vary in how picky they are; some are quite liberal and seem to take just about anyone with a pulse and a credit card, whereas others look more closely at whatever training you have had and your resume of sailing experience, yet will still want to get a on-scene feeling for your actual ability. This can also vary with just how ambitious your charter