What kind of design?
For industrial design (i.e. swoopy, organic shapes) something like Autodesk Fusion 360 might be best. It's a cloud based service very affordable and more of a "push pull" modeler
for mechanical design (machine design, etc) Solidworks is the most widely used in the industry
What age group of kids are we talking about? That would affect selection too... there are simpler programs like Sketchup, etc that might be better for young kids
If it's high school/college then a good thing to do would be to look at the job market in that area and what they are looking for. There's nothing worse then training students on systems that aren't in demand in the area.
