this will sound crazy but go to the local convenience store and get some heel liners. they will make your feet stick to the back of the shoe. also since there bots you could just fill the space with newspaper or something.
Since your feet swell more in the afternoon after walking in school or work, you should have waited to measure them then! But boots are cut different than your regular sized shoe so it should fit. I wear an 8 but I buy my boots in a 9 depending on how sharp the toe's point is! If its suede, you can dampen the boots and place them in the dryer in a pillow case ( same goes for leather but not in a hot temperature) One more thing, if all else fails, put on thin socks and then wrap your feet like a burrito with socks to make the boots feel full on your feet.
In the first place you should only wear shoes that fit. You have one set of feet. Treat them with respect and you will be able to walk on them when your old and gray. Start messing with your feet and you could end up in a wheel chair someday. It's just not worth it. Much like your eye's, don't wear glasses that you can't see out of clearly just because they look cool. You could end up blind. You have one set of eye's. One body only get the picture?
Put 1 or 2 pairs of innersoles in them or if you can get your hands on an old sheepskin ugg jacket cut it up for innersole liners. Cut the heel longer so it goes up the back of the heel. Trim it level with the top of the back of the boots. Once you know it will fit you, you can blutac it to the back of your boot so it stays in place. Also place blutac on the sole of the boot so it doesnt lift every time you slide your foot in & out. If your foot has stopped growing you can super glue the sheepskin innersoles in permanently all the way up the back of the heel. Cut up & poke sheepskin pieces in the toes of the boot. They are soft & won't hurt against your toes. If your boots are lace up tie your laces as tight as you can. If your feet still slip around inside your boots add thick sox. If still too big then I would suggest not wearing them. The outsides will buckle & crease up & spoil the look of your boots. Goodluck.
Look on line to see if any other place sell them... Contact them and see if you can do an exchange if you purchase the correct size & get a refund after they receive the other pair back into their stock. It's easier to do at the same place you purchased the shoes from... Also they'd have to be unworn. Some stores may charge a restocking fee but that maybe better than having shoes that may cause blisters or sprained ankles. Best Wishes