Block websites on Student iPads using Screen Time
You can block websites on student iPads on an individual basis using Screen Time.
Let's set up Screen Time:
1. Settings > Screen Time > tap on Content & Privacy Restrictions

2. Toggle on Content & Privacy Restrictions > tap on Content Restrictions > Web Content

3. Tap Limit Adult Websites > under "Never Allow" tap on Add Website (add desired blocked website)
. 
4. Let's lock down Screen Time using Screen Time passcode so students can't alter settings:
Tap Back to the beginning page > tap on Lock Screen Time Settings > choose 4 digit code > confirm 4 digit code > it will ask if you want to use an AppleID as a Screen Time Passcode Recovery - tap Cancel
. 
5. Now when the student attempts to go to the blocked site it will load this:
You can allow the website by tapping on "Allow Website" which will prompt the Screen Time passcode to be entered to proceed to the blocked website.