In Modal auxiliaries^x, Past simple^x

1. You must be pretty dumb if you get a bad mark. This is fine. It’s the first conditional: modal auxiliary verb + if + present simple 2. You must be pretty dumb if you got a bad mark. This would work as the second conditional if you replaced ‘must’ with ‘would’. In such a case, the...

We normally "reach", "come to", or "arrive at" an agreement. We don't "get" an agreement, at least not in the context you have given. Therefore the second sentence is correct, the first isn't. Because you are using the past-tense modal auxiliary "could", I suggest that you switch to the past...