It looks like it should be read 'ate ga hazureta' to me, but I could be wrong. There would be a 'ずれた' by itself it were to be read as 'wa', which is a bit strange, I think.
Edit: It's definitely read as ha. The sentence chose to leave はずれたin hiragana, but 当てが外れた is how the whole thing looks in kanji, eliminating confusion.