The awesome people at Powtoon have just released a brand new slide presentation tool, appropriately named Slides. You can sign up for a free account through your Google, Facebook, or Linked In account. Although the free version does not allow you to download your presentation, you can get a link to the presentation, or grab the embed code.
Slides provides beautiful, professional quality templates in a variety of designs, which can also be customized in the Powtoon Studio. There are eight templates available to you in the free version. In addition, Slides has a feature called "Clip-A-Vid", in which you can easily clip and embed any YouTube or personal video into your slide show.
Keep in mind that according to the Terms of Service, Slides is for users 18 and older. However, Powtoon does have a paid EDU version that is quite reasonable ($8 a month for a teacher and 60 students). See all of the EDU pricing options here.