I didn't know barbies were still prescribed, as benzos pretty much took over their function and were deemed safer.
I would not suggest taking them during an exam, as benzos are known for their memory-lapsing abilities, I might surmise that barbies do the same.
You can expect something somewhat euphoric, with a rather high addiction potential (generally speaking; that's one reason they stopped prescribing them so much and switched to other drugs). I don't know exactly which barbituate you have, so I can't look it up in my Rx manual.
Interesting, though, I didn't know they Rx'd them for migranes or related symptoms. I wonder how their mechanism of action works there...