Since you are asking, it is probably worse than you think it is. Imagine walking on the surface of the sun from the moment you wake up for rope drop until 11 pm - 12 am. August is doable, but you need a serious plan to make it work. I'm a teacher, so I don't have a lot of convenient times to go, either. I get it. My DH and I have been doing August the past three years, and we know how to make it work pretty well now.
Go as late as you can in August - the crowds get lower and lower the further into the month you are. We've done it in the past that we come back home the night before I go back for teacher work days/orientation. It's always very depressing to go back.
Wear light clothing - athletic gear is great. Make sure to have cooling towels and wet them constantly. You can see one on my DH's shirt in my picture. Mine blended in a little more - it's more like a "necklace." Bring a refillable water bottle and fill it with ice before you set out for the day. It won't take long for it to melt. Get more ice and water constantly from kiosks. You need to drink water constantly or you WILL get sick. I've gotten heat sick twice on our trips, and it was miserable.
I see you're in the Chicago area, too. Certainly, there are hot days in our area, but it's NOTHING like what you'll experience. Walking several miles during the heat of the day during the days leading up to your trip might help to acclimate you, though.
Afternoon breaks are incredibly important. You won't be able to stay out all day - noooo way. Wake up early for rope drop, get in some attractions, and move on. Take a nap in the AC or go to the pool at your resort, and come back later. Since you're a veteran, you've probably seen just about all of the parks at this point, which is good because you probably won't get to do everything. We've taken breaks in hotel lobbies when we didn't want to go all the way back to our resort. (The Swan/Dolphin lobby was so nice in between mini-golf and HS...)
Try to stagger outdoor queues with indoor queues. Make Fastpass choices based on the attractions with long outdoor lines so that you don't have to deal with them. For example, in MK: Splash Mountain, BTMR, Seven Dwarfs. It's tempting to do Space Mountain, but trust me, you will be so thankful for the A/C...
I know this probably sounds dramatic, but I'm just trying to be real about what it's going to be like. My first time in August, I didn't realize how bad it was going to be and learned a LOT. We're August veterans now, so trust me when I say you can do it, but have a GOOD plan that is based on getting breaks in the A/C. Don't be disappointed if you or someone you know needs to call it quits for a while because they're drained due to the weather.
I'm happy to give more advice about August if you have more questions!