Too small projection areas in too large a space and high resolution projectors suffer. If your projection area is a sensible size, if you can read it on screen at that resolution, it'll be fine to read in the projection too.
Mostly, classrooms aren't an issue, they're not massive spaces. More of a point to watch for in assembly halls and such.
As WeirdFish says, cost. Also, higher resolutions projectors might also start requiring HDMI not VGA (although so might your staff laptops!) which might mean replacement cable are required and HDMI suffers when pushed to 10m or beyond, VGA is surprisingly resilient. Replacement cables mean extra cost, so something to check out and plan for.