Have you ever been sitting in a class at the masjid scheduled around the prayer time? Like, it lasts until asr, or until maghrib, for example? It tends to happen a lot, I think, since the call to prayer necessitates a break in the class and makes for a good stopping point. More importantly, it gives you the advantage to pray that salah, at least, in the masjid. That is, if you don't have a habit of performing prayers in the masjid, it gives you a reason to do so.
In my time as a Muslim, I've learned that it's not appropriate to leave the masjid after the adhan is called unless you have prayed with the congregation. Why? Here's why:
Abu Hurairah (R.A.) said, "The prophet (S.A.W.) ordered us, when we are in the Masjid and the Salah is called for, not to leave the Masjid until we pray." [Ahmad]
See? And when I took the Fiqh of Salah class from AlMaghrib I learned that you should pray that salah with the congregation even if you've already prayed it! (For example, if you're travelling, or if you follow a different opinion for asr.)
But I have at times been sitting in a class or halaqah which lasts until a prayer time--at which point nearly everyone gets up to leave. I'm not sure if I think it's bad timing or bad etiquette--I'm sure that they don't know about this hadith, or else they might be inclined to stay. After a few years, I've learned to adjust my schedule to the prayer times, and absent any extenuating circumstances, will stay for the congregational prayer if I am there for the adhan.
But what's the best way to tell people, when they are getting up to leave, if they figure that the class is over and they need to get home. Will a few more minutes (20-30 max) make that much of a difference, at 9:00pm? Do many people just not know about this hadith and etiquette? I wonder, and also wonder the best way to share it. Thoughts?