This is a great way to get around students being able to get directly to youtube. If there is a video you want them to be able to access, you can simply download a copy of the video and put it on your google classroom. Here's how:
1. right click on the video you are trying to export and select "copy video url" . (to right-click on a mac you can hold down the ctrl key and click your mouse)
2. go to keepvid.com and paste the url.
3. select download mp4 -720p
4. when the download is finished, add the video to your google classroom by clicking the paperclip to add a file and selecting it from your downloads folder.