Like the person who responded before me, I'm not sure if you're still working on this assignment (probably not, based on circumstantial evidence).
You could take the well trodden path of defining what a hero is to you and write your speech accordingly.
Or you could dare to say the opposite of what would invariably amount from the first option by stating emphatically that there are no heroes left in America and launch a vicious, partly humorous tirade on the decline in virtue and morality if you so feel that is the case.
Or you could do a time series analysis that equips your argument answering yes, but not as many as we used to.
You could also make the offbeat assertion that we will always have heroes in a substantially similar proportion, just qualify it by offering that the meaning of the word hero will shift to reflect cultural values in a state of constant adaptation -- whether the trend has been good or bad in recent history you can decide by illustrating the qualities of heroes today and heroes of yesteryear and concluding something interesting.
It's a dull topic and I'm sure the submissions will have predictable ideations.
Fortunately, for your grade level you're writing to no specific end and so you can come from the rafters or pop your skull out of a burrow without ill effect.
Mustafa