As someone who also deals with the "get out of the car fast, it's the drop off line!" I would strongly suggest a LATCHable booster. Any time saved by switching him out of a harness into a booster is going to be negated when he has to rebuckle the booster behind him.
(I pooh-poo'd the necessity of buckling boosters until I was in an accident earlier this year, and
saw what happened to the unbuckled booster. (Pics at the link.) I am now neurotic about always having them buckled in.)
I wouldn't bother moving him to a booster unless it's a LATCHable one, just out of experience in regards to the time factor. :twocents:
Also, do you have the option to drop at a back entrance or on a side road, where he can take his time and you can then watch him walk to school? I've done that with my kidlets and it helps a lot with the time crunch issue.
Although I understand Darren's point in that blog, I've also found that harness to backless booster is a HUGE difference, unless the child has been booster trained first. Even if there's a negligable safety difference in using the back, the back is useful for training the child to stay in position. Once your son is used to being in position, I'd be a lot more comfortable with him using a backless. (I've used a backless with my well-booster-trained 6.5yo on occasion, for the <1 mile drive through the neighborhood to school -- although I prefer the Monterey, I don't wig when he uses the backless, either.)