A group of dedicated friends or shooters are just as capable of doing some research and come up with a training program and associated practice regimine just as good as any school.
It depends heavily on the group of people involved. The statement above is like saying that any group of dedicated persons are just as capable of raising some money and starting a business as successful as any other business.
In theory it's true, but reality tells us that 90% of businesses fail rather than becoming successful.
A successful school will almost certainly do a much better job of training people than a group of dedicated friends/shooters who aren't staking their livelihood on being excellent trainers, who haven't proven their worth to the shooting community and their ability to train by staying in business for an extended period, and who aren't training a number of people on a regular basis.