How to play a movie on a MacBook using a multimedia projector for output?

At our school there are (not smart) BenQ multimedia projectors in every classrom, every one of these is connected to an AppleTV (AppleTVs are *not* connected to WiFi) via HDMI cable and more and more colleagues are telling me that there seems to be no way to play a video stream (or a movie from the media library they own) containing image and sound via AirPlay or even via HDMI (using the official Apple adapter) now: the sound is perfectly fine, but there is only black rectangle instead of video even if the MacBook is connected directly to the beamer via HDMI. So my question is: is there a way to stream a movie without using a Windows laptop? YouTube (not YouTube Movies!) works perfectly fine btw. Are our (quite) old projectors causing the problem? **Update:** Looks like it's a DRM problem. VGA connection (via Apple adapter) works perfectly fine: the movie and its soundtrack are there.