I bought a small Logitech Z515 Wireless (Bluetooth) speaker a few months ago. I’ve got it in the office today, and have it hooked up to my PC.
One thought would be to take it with you and have a class outside. If you can figure out how to use your mobile device (iPad / Android phone, etc.) as a microphone, then you could put your wireless speaker at the “back of the class” and that would make it possible for your many students spread out under trees, etc. to hear what you are saying… not just those up front. I guess you could use this indoors for the same reason. Place the portable speaker at the back of the class to help students there hear as well as those at the front of the class.
ADDENDUM: How disappointing! I tried various apps, both for iPad and my Android phone and none of them worked, or worked along with pushing the audio via Bluetooth to the wireless speaker.
It would be nice to be able to use your iPad to view notes while giving a lecture and using it’s microphone to push audio out to (one or more) a wireless speaker (via Bluetooth).