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)

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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 

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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.