I can't help you as I'm not a teacher but I just saw your question an googled it finding this interesting article "A historic" or "an historic"?.
CSI is showing a trailer for a program where it flashes up the words: AN HISTORIC TELEVISION EVENT Isnt this incorrect as written and how on earth does that get past a proofreader?
Actually, it is correct. In this instance, it is "an event", so it is "an", not "a".
Your comment of how does it get past a proofreader, however, is quite possibly the funniest thing I've read for a while. The English language is butchered every single minute on television. There are days when I am watching I end up yelling at the TV to try to make them realize the absolutely stupid way things are written.
I know, it's only going to get worse, not better!
By all logical and traditional rules of grammar and pronunciation, "an historic" is definitely incorrect usage, but yet it occasionally makes it to print. Perhaps the speaker/writer thinks it sounds "posh" or something, I don't know.
As far as "how does this get past the proofreader," you'd be amazed at the result of what I refer to as the "dumbing down of America"... I work as an editor for a magazine, and have received articles written by people with advanced degrees from prestigious universities which are riddled with errors of fact, grammar and spelling. I note that the similarity between these authors is that they all graduated sometime after the mid-1990s, which means that they attended high school in the era when academic standards were lowered in order to assure that more students passed, and in which time inflicting (previously) standard English grammar upon students was considered racist and elitist.