Assume you're talking about the radio button field option "Enable Choice Limit".
You say this is a bug; in what way do you conclude that? I'd say it is Machform working as expected.
I would see it as an improbable situation that you have both students that submit at exactly the same time, so Machform processes both submission requests at exactly the same time; it might be milliseconds in difference but one will be ahead of the other, and the second one will not succeed. You're expecting both entries to be counted as "one" ?
The only work-around I could suggest, is increasing the quota for that particular field. Say you expect 100 entries, then make the quota for that field 101, or something like that.