Good Friday and Easter took priority over Earth Day, so now that the holiday and time with family is over, here's an awesome Earth Day article about bottle schools.
Basic premise - don't cut down trees or dig up materials from the earth to create buildings; stuff small soda bottles with your trash and create walls (a bit "Tetris" in style). I suppose the buildings may not be the most beautiful if not covered with cement to hide the bottles, though I would guess that some sort of tinting or stucco could be used to solve that problem. I'm not sure how insulating trash-filled bottles are, but for a cost of less than $10K to build a two-room school, I don't think a little insulation will throw off your budget.
Years ago I worked as a counselor at a day camp. We made a raft for the pool and a small bench out of bottles. The raft floated, and the bench was strong enough to support my seven-year-old campers (though it wasn't very comfortable). They didn't stay together as well as the schools are holding up, but we weren't engineers or contractors, so we'll blame any design flaws on that.
Mother's Day and Father's Day are both coming up. Know someone who wants a shed for the backyard (or a raft or a bench)? I'd be happy to send you some bottles!