Yes, keep it out of the heat and sun. I'd keep it in the coolest place reasonably possible, if I were you. Heat, light and oxygen will quickly break down nutrients in the feed, rendering it less nutritious (ie, tell your parents they are wasting their $$) or worst case scenario it can go rancid and be mildly toxic to consume.
Outside, it should be stored under shade so that the sun never directly shines on your galvanized container. This is most important in summer, because it's hot. During winter, the galvanized can will block out light (as it will in summer), but the temps are so much cooler, that the heat issue isn't a problem.
Also, consider how quickly you go through the feed that is stored in the container. If it takes you 2 months or longer to go through it, IMO, you are storing too much feed in there, at least during summer months. Even a garage is going to get pretty warm. Buy/store just enough feed that you will use it all up within 2 months, assuming it's being stored properly as mentioned above.
Something I do since my stored feed is further from the coop than I'd like in terms of convenience of refilling their feeder is to keep a bucket by the can of feed. I keep a scooper (a quart yogurt container) in the can of feed and scoop out enough feed into the bucket to fill the feeder. Then I can tote the bucket of feed to their feeder and refill it easily.