Synth is an excellent tool for creating short podcasts. In fact, it's one of my picks for Best of the Web for the 2018-19 school year. But to call Synth a podcasting tool is a bit misleading because it is more than that. On Synth you can create threaded audio conversations in which people reply to your original podcast with audio messages of their own.
Synth has just released an update that introduces podcast moderation. This means that you can now create private podcasts and have students reply with private comments instead of having all replies be private.