Restricted Mode on YouTube is designed to prevent pupils accessing potentially unsafe materials. It is on by default, but can be turned off by teachers as it does sometimes categorise videos which are safe for PSHE as restricted.
To turn Restricted Mode on/off on youtube, go to the three dots at the top right, then click on Restricted Mode:
You may need to sign in to Google for this option to work, I've had varying success with it signed in or out.