Where are you checking for flights? Have you tried the discount shops like dial a flight or ebookers and done the usual google search on "cheap tickets to Florida"?
As wannabe said the cheap tickets usually involve flying with an American airline to one of their hubs (eg: United LHR to Chicago) and then grabbing another flight to Florida. Then again deals can often be picked up with others like Britannia etc.
Also checkout teletext as they often have good deals. You could always "buy" a weeks holiday and only use the flight portion of the fare to get you to the US.
If you want to get creative you could always try for cheap tickets to say NY or Chicago or anywhere else for that matter in the US and then find a cheap internal flight from there to Florida. Assuming you are heading to Naples you might get a flight closer to the school than say Miami. With the exchange rate it might save you some money as well.