Unlike nukes, antimatter weapons can be made small enough yeild that you don't flatten the whole of hogwarts when you use one.

...but the amount of antimatter you'd use for that is very small indeed; about 0.000023 grams for an explosive yield equivalent to 1 ton of TNT, going by Wikipedia's numbers. I wouldn't put it past Quirrell to figure out a way to transmute quantities that small, but it'd be tricky; everything we've seen transmuted onscreen has been macroscopic.

Also, all the initial yield would be in the form of highly energetic gamma rays, so we'd likely be looking at something more like a hard radiation pulse than a bomb.