I do this for my daughter,...sort of. Her birthday is in December, which is the school year, but winter birthdays are limited as far as activities being indoors. We also usually have limited number of kids at her birthday party because our house is small and can't accomidate a large number of kids running around. Alot of her friends are left out unfortunately.
So, last summer, we tried something new and rented a moonwalk for a day. We invited everyone we knew. We had a hotdog/bun machine going all day, chips, etc and a cooler of beverages and people helped themselves. (This was pretty easy for us because all we had to do was fill up the cooler and toss hot dogs and buns in the machine. (We even used all the extra paper plates and napkins we had from various events...halloween, birthdays, christmas, etc. We also had a bunch of her outdoor toys available so if kids wanted a break from the moonwalk, they could do oher stuff too. It was a BLAST and we plan on doing it every year.
I'd recommend it...even if it's only a month of so after school starts, but I'd stay away from the dead of winter. I've heard some of my parent friends say that their kids will suggest postponing their own party for this reason.
I thought I'd tell you this...we call our party an "End of Summer" party, so that way, kids are not obligated to bring a gift. She has her own party with gifts around her b-day. You might consider the same thing...call it, "School's 1/2 over Party". :)