This python script plays a beep sound with the player of your choice when your classes begin. You can change the times and add more as well. I've set variables for the player as well as the file to be played, meaning that you can make the script play whatever you want.
To install this script, just download the file named classnotify.py as well as the file named beep.wav and run classnotify.py using python (If your system does not have mpv, the script will exit with an error message. Make sure you have the player you are choosing to use installed and the 'datetime', 'time' and 'os' modules in your python install! If you don't want to use mpv you can just change it to another player, by changing the player variable in the script).
To install this script, just download the file named classnotifywindows.py as well as the file named beep.wav and run classnotifywindows.py using python (Report any issues, you encounter, with the Windows script, because I am unable to test the script myself to check if it is working). Make sure you have the "time" and "datetime" modules installed.
I do not have any experience with MacOS, but the steps would probably be the same as the ones on Linux. Just make sure you install the required modules as well as mpv or another player if you choose to change it.