If the teacher was really embarrassed and apologetic after you woke him or her up, then I'd let it slide, but if the attitude was "it's not a big deal" then I'd get another teacher.
How could it even happen, anyway? My teacher sits right next to me and interacts, sings phrasings, shows articulations an octave up from where I'm playing, writes on the score. I'd notice in seconds if he'd fallen asleep.