I do not think there is an issue with turning down a spot at FSP, provided that you have a good reason...missing your high school graduation sounds like a very valid reason to me; especially given the short notice.
If I was you, I'd contact DFR and explain to them over the phone as to how you genuinely want to attend and it's been a difficult decision, but due to clashing with your year 12 graduation it is something you had to choose over FSP. Impress upon them that you are most certainly keen to attend another FSP in the new year asap, and insist they make a note on your record as to the reasons why (high school graduation...if you say anything about how you're miffed with them for mucking you about, or that its pointless as you have to wait a year due to your arm - you'd be signing your own death warrant to your ADF application). Just my two cents.