I have been using NCSS in my high school class room for 22 years. I find it very easy to use and my students use it to calculate their results for their lab reports, science fair projects, and now they are using it for AP Research projects. It can be used for everything from the basics to the most advanced statistics. NCSS is very affordable for any high school budget.
I adore NCSS and PASS. I have been using them for 20 years now.