I love classes...all the average joe's wanting wanting to play in full kit, and all the professionals wanting to dress slick like the average joe...everybody's playing dress up.
I've become a fan of the below and a good rigger's belt and adding the shingles as needed for the class design (which they should give you).
https://www.blueforcegear.com/gear/belt
From my experience, it's not necessary (short of running single-stack in a pistol class) to carry an excess of mags. Do BRING enough and have full mags handy at your "staging point" near the range/line. If you're in a class where they've got you burning 5-6 mags in a few hitches...you're in the wrong place.
That is hilarious!!! And true.
Don't get the floppy belt the link sends you to, a rigger's belt is significantly stiffer, though you want to think about how you're going to mount your accessories and holster to it before buying. Zip ties if you get desperate.
I'll get shot for saying this, but buy a $30 airsoft or Condor or whatever cheapie chest rig that holds 3 or 4 mags. Run that for the class to get a feel for what you want/don't want in a chest rig. It'll solve your carrying issue easily and cheap for a non-serious application (if it rips in a class, no biggie- but I doubt you'll tear it up that quick). Ditch it later when you decide on what works for you and you basically rented it for $5 or $10
After the class and using it, you'll have a better idea of what you want/don't want for your future purchases. Expensive to get a Haley or other expensive chest rig to find out you prefer a different configuration. I've used cheap nylon webbing, tri-glides and cheap plastic buckles + $15 triple mag pouches with the elastic cord on top to make simple cheap chest rigs for new guys. When they're done, they just return them and I hand them their $20 back as a rental. Unfortunately, all 4 are currently out... I think they became sales... easy enough for you to do yourself. PM if you have ?s. Though it may not be as worth it financially to make one.
Once you decide what you like, consider a plate carrier (plates are heavy and expensive btw). Try someone else's first as you WILL be significantly slower at first if you don't train with it. I value speed over heavy protection personally, but it is an option.
+1 on what everyone is saying in regards to way more than you think you'll need in water, gatorade (3:1 ratio); drink at EVERY point the instructor takes his ears off, dehydration later in the day will suck; I bring a LOT of bottled water as people are stupid and don't bring enough and I like to douse my head and soak my upper body with water on breaks. I'm a sweaty mess anyway... may as well use cold water and evaporative cooling to my advantage
I freeze 1/5 of the water bottles in the freezer. on really hot days about 1.5 hrs laying out in the sun will melt them completely. those are the days your ice cooler is cool water because the ice melted so quickly (everyone keeps opening it all day long)
batteries, something to cover the back of your neck (I duct tape a large square black cloth to my ear pro band and it falls down to my shoulders), it don't always stay in place and I look ridiculous... but I don't have sunburn on the back of my neck at the end of a 2 day course in the summer sun
light snacks and beef jerky are good
a small folding table or kids' table and folding chair or stool is helpful on a range without benches
definitely bring quality lubrication and put some in your ejection port to wet your BCG well during lunchtime/break; or anytime your gun is running slow/sluggish (Rem Oil might not make it to lunch in a high round count class)
bring a "ammo supply bag" or an ammo can; this goes WITH you to the line (along with plenty of water) and is kept far behind where anyone might trip or fall over it. but you can pull fresh mags from it quickly (if you have many to spare, bring every one you have); if not then bring ammo and you can load your mags at the line instead of running back to camp
if you have a spare AR, bring it (not necessary), stuff happens no matter how reliable your primary is- though I would strongly recommend against ever loaning your gun to a stranger
wear a stiff belt you'd use for concealed carry with whatever rigid mouthed holster you already have that is OWB and whatever mag carrier you have, preferably OWB. I'd skip any rifle mags on the belt unless you know you will want to run that way. I like a 20 rd mag on the belt behind 2 pistol mags, but that's personal preference. even with the shorter distance off the chest, I can reload faster off the belt from all the pistol mag changes I've practiced over the years
never more than 4 mags on the chest, 3 is good for me and I'm a big guy
a med kit is great, but it can stay with your ammo can/ammo supply near the firing line instead of on your person (once you get your gear sorted out, I'd move it to your person but not needed for this course). most guys will have med supplies... the ? is whether they have any training.