I've been using the full version of TechSmith's SnagIt for about seven years now and love it. It's not an expensive program (I was lucky and won a copy at EdCamp Philly a few years ago) but can cost a bit when you need hundreds of licenses for your school. SnagIt just partnered with Google to offer an app and extension of their program which can be used with the Chrome browser and is linked to your Gmail account. Screen captures are saved to a folder on your Google Drive named "Techsmith". If you wish to share a capture with your students, you are given a shortened URL where the image resides. Please note that if you are used to the full SnagIt version, this one has its limitations; image size cannot be changed, one arrow, circle or square style is available, (a handful of colors) one font can be used and the image is saved as a .png. For most people, this will be quite enough. Here's a brief tutorial which I made with the full version of SnagIt. You can find both the extension and app at the Chrome Store.