Just kidding. Now to business.
My old bag featured an awesome backpacking moose. Epic graphics like this are hard to find (this one was from L.L. Bean), and I didn't actually get to pick the bag because it was a present for my very, very first birthday. That's right, everybody, I got my backpack of ten years when I could literally fit inside it! But by the time I graduated fifth grade it was a) falling apart and b) had a backpacking moose on it, which was for some reason was uncool in middle school. And so I wanted a new bag. I made do with an intermediate backpack of some sort for sixth grade, but by seventh grade it was clear that that setup was not sustainable, by which I mean a backpack with nothing but a giant compartment in the middle is a recipe for a stinking black hole of junk. Actually, that's inaccurate. A backpack that is nothing but a giant fabric holding tank is destined to become your locker in miniature, which if you're me means it turns into a stinking black hole of junk.
So I maintain the standard that messenger bags are always cool. This is perfectly fine except for one disclaimer: sore shoulders are not cool. If you're willing to suffer for fashion, which I totally was at the time of buying my bag, then go for it.
My epic satchel fulfills all the parameters of a useful, long-lasting school bag. It has been slammed into lockers, kicked around under filthy bus seats, crammed until the seams stretch, and washed (both by the machine and by just being left out in the rain to fend for itself for a few hours). It is admittedly hard to test this in a store setting, but anything waterproof is usually a good idea. Think bullet-proof, shark-proof, bear-proof, you-proof.
We have already established that a good, simple messenger bag is always in style. But if you are craftily inclined, make sure that it is also able to withstand your creativity. Mine has been stabbed with pins, sewn, unsewn, ironed, de-stained, and generally artistically destroyed. But it refuses to accept destruction! Just like any good bag!
And finally we return to the original reason why I had to ditch the backpacking moose. Make sure your bag is not only barely able to contain your classwork, but very capably handle your classwork, reading book, junk stash of things to take nowhere, and generally the 150% of everything. There's never enough room. Just make sure you're not setting yourself up for a years-long space race. This ain't the sixties.
So there you are! Tough, creative, and size. I couldn't fashion them into a clever catchphrase (TCS, SCT, CTS?) so if you're going bag shopping, you might have to write them down. But I hope it helps!
One final note: monogramming. Consider carefully before monogramming, even if it's free. Remember that if you do your last name, it will be unwillingly handed-me-down to a lucky sibling, if you do your first name, you will inevitably feel like a third-grader at some point, and if you have a weird name like Mayim (MY-um) you will be called May-eeeeeem by random strangers for the rest of your life. Nice try, strangers.
Anyway, if you want to monogram, be creative, but careful. Think long-term. If you take my advice, your bag's immortal.

