An impromptu speech is all about improvisation. However, this doesn't mean that you shouldn't follow any rules, learn any tips, and practice to make the process easier. Such speech has two clear goals - it helps to improve your public speaking skills and it teaches you how to organize the information quickly. Therefore, it requires the same kind of planning as any other speech.
You've probably delivered an impromptu speech at least once in your life even if you didn't know that. Remember that time when you were asked to give a toast to someone at their birthday or to tell the class how did you spend your summer vacation? Well, that was an impromptu speech. And while it might not have been pleasant to you, it's still important to learn how to give one - because there's a chance that you'll be asked to give such speeches in the future as well.
That's why high school and colleges are focused on teaching students to react well to such situations by assigning impromptu speeches to them. The approach to such speech could differ a lot, depending on a professor or a teacher. You might be asked to attend a speaking conference or to give an impromptu speech in front of the class. You might be allowed to come up with your own impromptu speech topics or be told about the subject of a conference or a contest, for example. You might be allowed to pick funny impromptu speech topics or to be prohibited to do so, and so on.
But no matter what the situation is, you need to stand up and give that speech when asked. Doing so could be terrifying to many students - however, you could make it much easier if you take some time to practice and prepare in advance.
Here's how you can do so.
If you know that your speech would be related to the subject of the class or the course, come up with some ideas beforehand. If you don't know that, you'll need to pick on that spot - but even in this case, it's important to come up with the topic that is familiar to you.
Keep in mind that an impromptu speech is usually about informing your audience of something or persuading them in something. Therefore, you can find some inspiration in informational and persuasive speech topics.
Once you come up with the idea of a topic, try breaking it down into a couple of pieces - just like you would do with any other type of speech. Make sure that the introduction indeed catches the attention of the audience and is informative at the same time. It also should contain the main idea of your speech or its purpose.
Then, move on to the body of the speech. Try including no less than three statements into it, and be sure to support them with arguments and examples. Of course, it isn't always possible to come up with good ideas when you need to improvise - but you should practice doing so nonetheless.
Wrap the speech up nicely with a strong conclusion. Remember: a conclusion is basically a summary of your speech that, however, doesn't repeat the speech word to word. You need to make it unique and powerful at the same time, ending it on a positive note, including a quote or a powerful message, etc.
After you come up with some good impromptu speech topics and try organizing them into a speech, you can start practicing. Always keep in mind that you won't have much time to practice and to give your speech, so it's important to act quickly.
If possible, write notes - just be sure not to make them too detailed: you won't have enough time to write everything down. Try giving your speech in front of the mirror and focus not on the way you look but on how confident you sound and how well the speech goes.
Summing this up, an impromptu speech is usually unplanned and reactive. Therefore, it isn't always possible to come up with great impromptu speech topics for high school or college right away. To make everything easier, you can try practicing such speeches at home and focus on maintaining the structure and on presenting the information well rather than on giving a flawless speech and raising important topics.
Sure, you can still try coming up with a good topic. If you do want to do so, focus on persuasive speech topics and on informative speech topics. Pick the one that seems the most appropriate at the moment, take a couple of minutes to prepare - and start talking.
Don't worry: while an impromptu speech isn't the easiest to give, you'll master it with time. Just don't forget to practice and try picking the topics that seem challenging to you. This way you'll be able to learn how to face that challenges effectively even before facing one in the classroom.