Canva has premade templates that allow to make designs for flyers, posters, invitations, Facebook covers, business cards and so much more. It's basically drag and drop, so it couldn't be easier! For the most part Canva is free, but if you want to use any of their premium elements you'll have to pay $1 for each one you want to use. You can also upload your own images to really personalize your designs.
Canva has just announced that are launching a Canva for Work branch of their website. Right now I'm still not clear how it's going to be different, but who doesn't like to get in things first, so I signed up to be on the list for the beta release (you should too!)
You can use this program to make posters and decorations for your classroom or students can use it for presentations. You do need to log in, so if students don't have their own e-mail addresses I would suggest you make a generic e-mail address that you can sign up for a an account with. This way students can log in with that account and you always have access to what they have created. I have done that for my classes with a simple Outlook e-mail address.