Use a Proxy Site
Search for "proxy site list" in google. It is possible that that most sites that you find in the search results are blocked as well at your workd or school. Tip: Build a list of proxy sites at home. That will help you to find more working sites.
Make your phone to a hotspot!
Do you have a smartphone with tethering enabled through the carrier ? Then you are able to use this method. Using your mobile phone as the network connection will bypass all restrictions placed on the network at work or school.
Download the video
If YouTube is blocked then search for video you want to watch through Google. One of the first search results should be a link to your video. Now do the following steps:
First: Copy the Video URL
Second: Find websites that allow you to download videos from YouTube. Search for "youtube downloader"
Third: Enter the video url in the text box of the video downloader website and wait some seconds. The downloader will prepare your video.
Fourth: Download the video and have fun :)