Some manufacturers don't sell a site license. Also, inquire about the terms of the site license. A site license should offer excellent value. Schools are very conscious about the ethics of copying software, and if a producer doesn't offer site licenses or quantity pricing, look for another product. A good software product is a work in progress.
Manufacturers demand a lot of the product and a lot of themselves.
They should be available at low cost when new versions of the software are released. It's reasonable to expect that a manufacturer can't continue to provide free phone support for your problems as the customer base continues to increase. But you might look into the terms of tech support. Or, if there is a charge, know the rates. Don't expect free or low-cost support if you haven't taken a few minutes to complete and mail in a registration card.