If you can get an education discount on a MacBook, is it worth waiting for the back to school campaign?
Yes. Filler words so as not to violate the MacRumors Rules prohibiting one word posts.
can you just get a further reduction off the MacBook instead of getting a store card?
Not if purchasing from Apple. Sometimes when purchasing from Best Buy they will price match by taking the amount off of the price of the computer.
So is the only advantage of waiting until back to school the fact you get a store card you can spend on things in store?
Yes you are correct. Historically when Apple started giving out the store card it can only be used in the iTunes Store which includes the iOS and Mac App Store.
Is that worth it?