Totally agree with the posts above - take the promotion, apply to grad school for next year and use this time to get the management/job experience that will help you get the most out of grad school (and make you a much more valuable contributor/classmate!). I did an IR masters at 24, and although it did give me a head start on my career, I definitely didn't get as much out of the coursework or networking as I would have with a little more experience. Also, if you're dead set on the foreign service, start studying for the written exam now. You don't have to have a masters to get into the foreign service, and the timeframe from taking the written exam to actually getting a posting can be around 2 years or more (if you pass!). A masters will certainly enhance your skills and knowledge but there's always the opportunity for a mid-career degree. If what you REALLY want is the foreign service, start that process now.
Not sure if it's purely office speak, but a "gentle reminder" is anything but that.