Actually, "hit-and-miss" is incorrect in this situation.  From [this English Language Usage question](https://english.stackexchange.com/questions/209818/hit-and-miss-or-hit-or-miss) (emphasis mine):

>Hit **and** miss refers to multiple tries.

>Hit **or** miss refers to a single try.

In this case, if the exam can only be taken once (which I am assuming to be so), then it would be "hit-or-miss".

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The definition of hit-and/or-miss, from [TheFreeDictionary](https://idioms.thefreedictionary.com/hit+and+miss) is:

>Sometimes good or successful, sometimes not; having mixed or unpredictable results; random, aimless, careless, or haphazard. (Hyphenated if used before a noun.)

while the definition of touch-and-go, also from [TheFreeDictionary](https://idioms.thefreedictionary.com/touch+and+go) is:

>Extremely uncertain as to the outcome of something. (Hyphenated if used before a noun.)

The main difference is that "touch-and-go" is for an event that will *always* be uncertain, so in context to the question, it means that it is unknown if Ben will pass this exam.  "Hit-or-miss" in the same context would mean that it is random and unknown whether Ben will pass or not.  Just remember the difference between "hit-and-miss" and "hit-or-miss".