If you're having problems accessing certain websites in general, please refer to this troubleshooting guide. However, if you're having specific problems accessing Google sites and YouTube, here are some tips to help you identify and resolve the issue.
Cannot access YouTube
STEP 1. Check if you have any rules (on your device as well as globally) that block domains used by YouTube, such as youtube.com, googlevideo.com, youtu.be. If you do, remove or pause them.
STEP 2. Check if you have any rules blocking "All Video Sites". Domains used by YouTube such as googlevideo.com and youtube.com are included in this category by default. If you do, remove or pause them.
STEP 3. If you don't have any blocking rules related to YouTube, check if you have Safe Search enabled (Main Page -> Family -> Safe Search). If it's on, tap on Platforms and turn off YouTube.
Cannot access Google Classroom or other Google sites
Many Google services require ytimg.com, a YouTube service, to work. If you can't access Google Classroom, Google login buttons, or other Google sites, chances are you have a rule blocking YouTube or "All Video Sites" (both will block *.ytimg.com). See the section above for details on how to unblock YouTube.
If you don't want to touch your existing blocking rules, the easiest way to fix this is to create an Allow rule for ytimg.com on the device that needs access.