It's lots of fun to try out different fonts. Sometimes it can even be a bit too fun. Considering there are thousands of fonts out there, you can end up making some pretty wacky choices for your presentations - and if you are tempted to do this, imagine how your students are going to feel. For this reason, it's important to choose your fonts very carefully. In general, it is easier to read a large amount of text when a serif font is used.
Designers often recommend using font styles that contrast with headers from the rest of the text.
If you want to use different fonts within the same presentation, it's best to keep it down to only two or three. Like so many things in life, you'll just have to experiment before you know what works best. When you are trying to decide which fonts to use, consider how they will look on screen. Other fonts are more suited to print. Think about which method of presentation you will use and test your font in that medium to see if it is legible.