For what it's worth.... I wouldn't worry too much about it getting stolen at school, unless your school has an above-average bike theft problem. However, the folder will be more secure for stashing the bike at home, when visiting friends, trips to the coffee shop etc.
I'd also go for the less expensive 20" folders, like the Speed D7 or Mariner D7. It will ride a little better, the difference in the weight and folded size is small, and it's less expensive.
Last but not least, if you live in a fairly flat area you could go for the 20" Boardwalk, not officially listed but iirc still available. It's a single-speed bike so it would kind of suck on hills, but only costs $200.
Does Dahon no longer offer a super-light folding bike?