I live in a very sunny part of California. Weather is usually good, but late spring, summer, and early autumn can occasionally have heat waves with weeks in the 95+ range (with a 100+ day a few times a year).
Summer practices are either held in the early AM or after 6 PM. If we can, we pile kids into cars (most of the upperclassmen can drive and summer practices are technically unofficial, so no school liability issues w/ having kids drive other kids) and go to nearby nature preserves where trails have enough shade to be cooler.
During the school year it gets a little dicier. If really hot days line up with workouts, we'll usually change the workout up a bit to accommodate for the heat (a hard workout in 70 degree weather can be an impossible workout in 95 degree weather--we're trying to train the kids, not hurt them). We'll rarely actually switch a workout day to an easy day, but we might slow intervals or cut workout distance. If kids are doing tempos on hot days, we'll set the tempo portion to end at or near fountains. If we're doing track work, we'll have water coolers out there. We also encourage cross training on hot recovery days--I'd rather have kids cross train and be rested for workouts than run and be exhausted the next day.
With workouts, we've found that the key is to not hesitate to pull kids if they're looking awful--anyone can talk about how tough they are, how they ran workouts in 95+ every day in high school, and how we should make the kids run (etc. etc...), but I'd much rather pull a kid who's struggling too much than let a kid pass out from heat exhaustion.
All this being said, if you have a team run anywhere where it's hot, you'll inevitably have a few heat-related incidents, and we've had 3 or 4 in the last few years. One was during a major invite run in 95+ weather (think Clovis, Stanford, etc.), where a ton of runners had trouble and one of our kids collapsed. We've also had two kids faint due to heat during workouts. Just make sure you're able to provide (or otherwise have access to) adequate medical attention and water.