tōmys... "d" is neither a vowel nor a voiced consonant. see http://wiki.dothraki.org/High_Valyrian_Adjectives#Class_I + notes .
'D' is pretty much the definition of a voiced consonant. It's impossible to properly pronounce an 's' (unvoiced) right before it, that's why it turns into a 'z' (voiced). 'T' would be the unvoiced one, so if the sentence went 'five teachers ...', it would be 'tōmys tentyssy'.