It also depends what kind of classroom - is it a university lecture hall or a 1st grade class? If it's for kids you could have bulletin boards with crayon drawings (can find them on the web and print them upoin miniature) tacked onto them, maybe print out one of those alphabet plaques with block letters and cursive. Cheesy motivational posters here and there. Lots of little desks, floor tiles printed out and glued down. Maybe a chair rail around the room and a radiator, trash can… just brainstorming. A little bookshelf or 2. One of those roll down screen things behind the desk with big maps inside. Or just print up a couple of maps and pin them up. 10 years ago I would have said for a community college add ashtrays, but of course that doesn't work anymore..
I just googled Classroom, clicked on images, and there are plenty of examples. They even break it up into Primary and High School, and you could google University Lecture Theatres if you wanted more of a college look.
Most images I found have chairs/desks with tubular steel legs. I would probably use steel or aluminium rod for that. But if I didn't want to be bending metal rod, I would just go for a desk design that is made of planks of wood. Long tables would take less work to make than the individual combination chair-desk units.
Most of the blackboards are actually green - I wouldn't have thought of that if I didn't google it. They have a frame around them, and a ledge on the bottom to place the chalk and eraser. They never look totally clean, there is always some whitish smears from where stuff has been erased.
I agree with Strider, maps and posters and stuff you can print out at the size you need are a good way to cover a lot of wall and give a classroomy feel to it.
I vaguely remember a row of windows along one side in most classrooms, but that means more work painting a view of what would be outside, so you don't need to do that.
I'm trying to tell stories about my middle school years. About how I was the Wall-flower of my class! Mainly stories about me in English and Science class! Thank you nick!
if you use Foam board a word of warning that i found that it was hard to cut without shredded and bumpy-edged but you can get great results.
Good luck with it,