Julie, what are the most important characteristics of a lesson horse?
Here are a few things that spring to my mind when I think of the perfect school horse. First of all, SOUND! Although many of us use school horses with minor or chronic lameness issues, starting with a sound one is very important. On the same subject, good conformation is also important and both of these qualities lend themselves to longevity. I have a school horse that is 33 and we still use him occasionally for small children. He has been used hard all of his life as a trail and lesson horse and has never taken a lame step. If you want to keep your horses around for the long run, make sure you start with a sound and well-built horse.
Other issues that I would look at are age and gender. I would look at horses at least 13 years old; 15+ is even better because these horses have "been there and done that." Sometimes you can get really well-trained horses at a reasonable price in the 15+ range that have passed their peak performance level but still have many years left to teach riders what they know. You need horses with good life experience so that they are reliable in all settings.
I generally prefer geldings but I have had a few good mares for school horses. In nature, the mares in a herd are much more bonded than the male horses are and therefore mares like to form bonds with their riders and get burn-out changing riders all the time. Mares tend to test their riders more and can sometimes be more controlling and manipulative of their riders (these behaviors all have to do with natural herd behaviors). Geldings tend to punch-in and punch-out and do their job day in and day out, but a good mare is hard to beat and for that reason, we keep a couple around.
Temperament wise, I am looking for an insensitive, cold-blooded horse, at least for beginner's horses. I want all my horses to be as versatile and useful as possible, so I do not like to keep horses just for advanced riders. (See the article in this issue on Tricks of the Trade for a definition of cold-blooded and hot-blooded.)
Some other important qualities for a potential school horse are that he is well-trained and well-mannered. This will slowly degrade with beginner students and will take constant maintenance, but best to start with a good one. Also, it should be smooth gaited so that riders stay on a little easier and mid to low in the pecking order, so as not to constantly challenge riders. Finally, an "easy-keeper" (easy to keep weight on) is an important quality for your budget.
These are the things that immediately come to mind. Of course, there are many more factors to consider when selecting school horses and depending on your program, these qualities may be prioritized differently. Any new horse to your string should be given an evaluation period to be ridden and evaluated by staff only, before transitioning the horse into your regular string.