As with any investment a good way to reduce risk is to diversify. An old hand aeromed senior pilot advised me to do each stage of my training at a different school. You get to meet more people and experience more "flavours" of instructor so that if you get a dud who teaches it the wrong way you will realise it when the next shows you something different.
Schools want you to sign up for all your training to win your business. A good quality school will win your repeat business buy providing you with a quality service.
P.S. I have tried 3 schools but keep going back to my local aeroclub.