If you have problem accessing certain website in general, please refer to this troubleshooting guide.
If you have specific problem accessing Google sites and Youtube, here are some tips:
Can not access Youtube
STEP 1. Check if you have any rule (on your device as well as global rule) that blocks domains used by YouTube, such as youtube.com, googlevideo.com, youtu.be. If you do, remove or pause the rule.
STEP 2. Check if you have a rule to block "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 the rule.
STEP 3. If you don't have any blocking rule related to YouTube's domain, check if you have Safe Search enabled (Home -> Family -> Safe Search). If it's on, tap on the Platforms option, and turn off Youtube, see if it helps.
Can not access Google Classroom or other Google sites
Google Classroom requires ytimg.com, a YouTube service to work. If you can not access Google Classroom, chances are that you have a block rule on youtube, or "All Video Sites" (both will block *.ytimg.com). See above (Step 1 and 2) for how to unblock YouTube.
If you don't want to touch the existing unblock rules, the easiest way to fix this is to create an Allow rule (beta feature) for ytimg.com on the device that needs access.
If you can not access other Google sites or Google login buttons, it's likely due to the same problem.