Okay, here are some rough numbers for space planning, plus some explanation.
Basic formula: 40% for "group space" (student desks), 35% for individual/small group space (learning centers) and 25% for circulation. Storage space would have to be factored into each area, or be off the ground.
So 250' student space, 200' small group space, and 150' circulation/teaching space = 600 s.f.
A 24'x24' classroom could be built with 4' panels, and would give us 576 s.f. of space...
Each student needs 7-8 s.f. for a desk and chair space. If we have a class of 30 kids, that would be 210-240 s.f., plus another 25 s.f. for the teacher's desk space. (Current California initiatives encourage class sizes of no more than 20 for grades K-3, but I couldn't find anything about grades 4 & 5, so we have to assume that they might be bigger.)
My girlfriend, a certified elementary school teacher, says that 4-5 learning centers would be great, with 5-6 kids at each center during activity time. The teacher would direct one of the centers, and the others would be self-directed. Believe it or not, this actually works. But more that 5 centers is too many for a teacher to explain ahead of time, and would be too much to monitor at once. So if we put up to 6 kids at 4 centers, that's approx. 42-48 s.f. per center, for a total of 170-200 s.f. of small group space. 5 centers with room for 5 kids each works out about the same. If there are 30 kids in the classroom, one group could stay at their desks for a rotation, perhaps to do art activities...