There really is no collective name for a group of bishops in any general sense.
The Bishops' Bench/Episcopal Bench refers only to the long wooden seat in the House of Lords where they sit.
Of all the meanings of the word 'bench' offered in The Oxford English Dictionary none suggests it is a collective name for bishops.
Some might claim support from a long-ago quote by William Shenstone: "a little bench of heedless bishops"...but this was also a reference to an actual bench in an 18th century schoolroom, which is what he was describing. In other words, these were schoolboys who just might one day become bishops and their schoolbench could conceivably be later replaced by the Episcopal Bench in the House of Lords.
Consider practical applications, too. At a coronation or royal wedding, several bishops might be involved in the ritual, but the next day's newspaper would never say: "...a bench of bishops participated in the ceremony." Recently, too, there was controversy over the appointment of the Bishop of Reading because of his sexual orientation. It seemed that some of the Anglican episcopate - ie all the bishops of that Church - were for the appointment, some were actively against and others did not make their views clear one way or the other. Could we, therefore, have said: "One bench of bishops was on his side, another bench of bishops was not and a third bench of bishops didn't say"? Of course we couldn't...and it's nonsense to imagine we might!
That is, �bench' is simply not a genuine collective term for bishops in the same way that coven, flock, pride, school and a multitude of other such words are for the various creatures/people they are applied to.
Having said all that...'bench' is probably what you're expected to say!