Canon already suggests spells can be stopped by solid objects, but only if they're sufficiently solid. And powerful spells have been shown to blast objects, while weak spells haven't. It's not much of a leap. In HP canon, historical wizards may or may not have worn armor of some sort, but for an adult wizard, armor is probably more trouble than it's worth. Considering how versatile a properly trained wizard can be in combat, it shouldn't be able to do more than force the opponents to slightly revise their tactics, while increasing the wearer's fatigue.

Remember that these are first years. The difference between the quality of armor they and the government can procure is much smaller than the difference between their combat ability and those of aurors or Death Eaters. If they didn't have such a demanding teacher, they would probably be incapable of anything resembling proper dueling at this point.