Sometimes the Facts are Irrelevant
According to this article, 74% of Israeli Jewish high school kids say that Arabs are “unclean,” and 57% of Israeli Arab high school kids say the same thing about Jews. There are a lot of meaningful things that could be said here (all of them depressing), but surely none of them have anything at all to do with the factual question of the relative personal hygiene habits of the two groups.
Doug S. has it. "Unclean" has a distinct religious meaning to both cultures. I think most modern Westerners are unaware of it. Pigs and shrimp are always unclean. People may be permanently unclean, or only temporarily so, depending on such details as lineage, leprosy, menstruation, and nocturnal emissions.
The students on both sides could be taking "unclean" to have a specific technical meaning related to proper observation of religious rituals; thus, it is not necessarily surprising that practitioners of one religion view the other as "unclean."