How does a wireless device alert the master to its presence without interfering?
This depends on the protocol in use. But typically there's a teacher-student broadcast channel, whether that is a dedicated frequency band as in GSM, or some specific time-frequency slot, where it's desired that new students just shout "here I am, I'm CodeMonkey!".

After that phase the teacher says "OK, I heard CodeMonkey and Maria are new."

If you're not on the list but tried to shout your name, then you know you didn't get through, and will typically retry later on – often with something like "not going to try for a random number of announcement periods" (random backoff), to avoid making contention worse.