Hello Fitgirl50,
Thanks for visiting the Apple Support Communities. It looks like you are having some issues with Apple Music cutting off your songs midway. We certainly understand how inconvenient this must be while teaching.
1 - Does this only happen when connected to the JBL speaker? If so, have you checked to see if there are any firmware updates for the speaker?
2 - What is the current iOS version installed on your iPhone 12? You can find this under Settings > General > About > Version.
3 - Does this continue to happen if you Restart your iPhone?
After restarting, if you still see this behavior let us know.
Have a great day!