This site has a free or a premium option - I only use the free options. There are a variety of topics you can choose from - including an "Education" option.
When you find a template you want to use, you just have to select the orange "Google Slides" button under "Download this template".
From there, a new window will pop up and you can choose to "Make a copy" of that template. Now, that template can be found in your Drive and you can use it as you please.
Cool Features: the slides include descriptions of what and how to use the different slides, there are extra icons you can use to customize the slides, the specific fonts and colors are also added to the slides.
*If you plan to use multiple templates, you will need to sign up or log in. That will allow you to download 10 different templates each month.