Historical buildings have not been built with any sort of wind rating; however, any structures built now have to be rated to withstand a wind-borne debris rating. This is more for hurricane wind protection rather than tornadoes. The standard is a 150 mph wind-borne debris rating regardless of where you live in Florida. Many of the newer homes and schools are built using concrete masonry units (CMU, or concrete blocks) with a stucco finish, because those structures historically have better luck surviving major hurricanes with minimal damage; also, those structures are more mold and moisture resistant, important in a state with year-round humidity and potential storm surge/flooding. The schools that fared better during the hurricane had the CMU construction already.
Now, being from Tornado Alley originally (McPherson County, Kansas), I will say this. The best protection during a tornado is a basement, and tornado sirens that go off when a tornado or tornado system is in the area – these features are ubiquitous in Kansas. Basements just aren’t that common in Florida because of the relatively high water table. You will be surprised that storm sirens are not common here. I was also surprised to learn that Florida actually has more tornadoes annually than Kansas does. Kansas is behind Oklahoma and Florida in the average overall number of tornadoes that occur annually but unfortunately leads the nation in the number of F5 tornadoes that have hit the state historically. F5 tornadoes possess rotational velocities of 261-318 mph.
If you look at the historical paths of the tornadoes in Florida from 1961 to 1990 cited from the link above, all but about four that I can see are in the F0-F2 categories of the Fujita Scale, meaning winds up to but generally below 157 mph. Most structures (aside from mobile homes or buildings built prior to the 1980s) should be able to handle this because of the hurricane structural wind rating. The average number of tornado deaths per year is quite low when you consider the population of Florida; however, there probably wouldn’t be any tornado-associated deaths per year on average in Florida if there were widely available storm shelters and siren warning systems. I am a believer in the storm sirens, especially when storms occur at night and in areas with mobile homes and older homes because tornadoes hit quickly. Several times when I have been in a tornado warning area, the sirens gave us only a few minutes of warning, but it was enough to find shelter. Mobile home parks are leveled on a regular basis in Kansas (there was one in my hometown that seemed to CONSTANTLY get hit), but the sirens go off and the occupants drop what they are doing and go to a below-ground storm shelter, so fatalities are rare.
Like most places Florida has a very wide mix of structures, from mobile homes to tract homes to large office towers. Building codes were updated after Hurricane Andrew in 1992 to require roofing struts and foundations to be securely connected to frames. These improvements have helped newer homes to better withstand sustained hurricane winds of 100 mph or more, but tornadoes are more of a rarity here. Their sudden, rotating high winds and pressure differentials can rip apart most any building.