This question has no simple answer. As a professional daydreamer and as someone who has spent a fair amount of time studying the process of creativity, I can tell you that, in a nutshell, creativity can't be lost. Like our bodies, it just gets soft and flabby if you don't take care of it.
First off, let's get this out of the way, some people aren't creative. Their minds don't process that way and going to science class could not have robbed them of something they never had. I consider myself very creative, but I waved goodbye to anything math-related long ago, simply because I didn't have the desire nor the affinity to pursue it... I suck at it is what I'm trying to get across.
That being said, structure, in my opinion, is creative poison. Whether it be school, work, or anything that locks you mind down without allowing it to explore the fantastical. I'm living proof, I stopped writing for a few years while I worked in a professional environment, and I had a bit of an adjustment period when I started up again. Creativity is no different than body building, cycling, pottery, kung fu or whatever else you can think of, if you don't practice, it will eventually hamper your skill. It;s not gone it's just rusty.
So that was my absurdly long-winded way of saying practice makes perfect. If you have it in you, you can't lose it.