If they insist on activating it for you, tell them that you can't activate it yet and don't have time to configure the phone for a few hours due to classes. If they push back (they shouldn't), then tell them it's for another family member and that you can't deactivate their phone yet, that you are showing up early to purchase the phone before you're ready to use it, by which it may have gone out of stock.
Again, they shouldn't bother you at all, but if they do say the first line, and if they still act like jerks say the rest. Speak to the manager if your clerk refuses to leave it unactivated; VZW has no policy that requires them to activate the device for you and imposing this upon you also leads to reduced customer convenience and satisfaction which is something they don't want to deal with.