Are those one-on-one teaching sessions? There are many encoding/transmitting steps involved in a screen capture/sharing session, so I'm not surprised that the latency isn't good enough for real-time music.
I've never used it before, but
Internet MIDI from TimeWarp Technologies sounds like it was made for exactly this situation. You could still use Google Meet, but your MIDI messages would get there much faster than the video.
The idea is that your student would see a new MIDI instrument on their computer that they could use in Synthesia. But the notes coming in would actually be from your digital piano.
Again, I've never used it so I don't know a lot of the details like whether it works in both directions (or if that would even be helpful). And it does look like both you and the student need to buy a copy, which sounds a little expensive just to try it out. But, it is a tool designed specifically to solve the problem you're running into, so maybe it'll be helpful!