I'm a substitute teacher and I wear my knee-high boots to work all the time, with pants or with skirts. I think it's fine. (My boyfriend thinks knee-high boots make girls look "easy," as well as those backless tops, but hey, I didn't stop wearing them.
Well, the boots, anyway...backless tops remind me of freshman year.) Mine have quite high heels, but you can also get them with lower, chunkier heels if you're concerned about looking too tarty.